Scott Baker: 3.47 ERA with 68 strikeouts and 14 walks in 83 innings

A rough 2006 season left Scott Baker at Triple-A to begin last year, but he emerged as a dependable mid-rotation starter after rejoining the Twins in mid-May and has now become the team's default ace with both Johan Santana and Francisco Liriano in New York. As one of baseball's most extreme fly-ball pitchers, Baker will always struggle to keep the ball in the ballpark, so his serving up 13 homers in 14 starts this season comes as no surprise. Despite that he has the rotation's best ERA at 3.47.

Baker has increased his strikeout rate while maintaining pinpoint control, joining Roy Halladay, Josh Beckett, Cliff Lee, James Shields and John Lackey as the only AL starters averaging fewer than two walks and more than seven strikeouts per nine innings. Since returning from a month on the disabled list with a groin injury Baker has a 3.06 ERA and 39-to-9 strikeout-to-walk ratio in 50 innings, giving him a 3.97 ERA and 170-to-43 strikeout-to-walk ratio in 227 innings spread over 37 starts since last year.

Nick Blackburn: 3.65 ERA with 64 strikeouts and 18 walks in 118.1 innings

Baseball America drew criticism from me (and others) for ranking Nick Blackburn as the Twins' top prospect prior to the Santana trade because, as a 26-year-old with a poor strikeout rate and mediocre minor-league track record prior to last season, he looked to me like "a back-of-the-rotation starter or middle reliever." So far Blackburn has made BA look smart while out-performing my expectations considerably, tossing 118.1 innings with a 3.65 ERA and 64-to-18 strikeout-to-walk ratio in 18 starts.

Blackburn's breakout between Double-A and Triple-A last season looks like legitimate improvement, although maintaining a sub-4.00 ERA will be difficult over the long haul with such a low strikeout rate. He's made up for the lack of missed bats by walking fewer batters per nine innings than every starter in the league except for Halladay and Mike Mussina. Interestingly, the combination of few strikeouts and even fewer walks has made Blackburn a near-perfect fit as Carlos Silva's replacement:

Eerily similar. Blackburn has slightly more strikeouts and slightly fewer grounders, and their walk rates and opponent's batting lines are nearly identical. Blackburn has even followed in Silva's footsteps by establishing a reputation as a "ground-ball pitcher" without actually inducing all that many grounders. As far as compliments go "the next Silva" may not seem like a great one now, but he had a 4.42 ERA in 774 innings with the Twins and Blackburn looks like a younger, cheaper and perhaps better version.

Kevin Slowey: 4.26 ERA with 61 strikeouts and 13 walks in 82.1 innings

Kevin Slowey's had an up and down year to say the least. He left his first start in the fourth inning with a strained biceps, spent the next month on the DL, returned with a 3.38 ERA in six starts, got rocked by the White Sox, bounced back to go 3-0 with a 0.93 ERA over his next four outings, and then struggled in his final two starts of the first half. And now he's questionable to make his first start of the second half because of a finger injury. Much like Blackburn and Silva, within that he's been very similar to Baker:

Nearly identical numbers across the board, save for a 50-point difference in slugging percentage that can be traced entirely to Slowey allowing twice as many doubles as Baker. They're both extreme fly-ball pitchers who have excellent control and miss an above-average number of bats. In other words, for all the talk of Slowey lacking a supposed "out-pitch," he's essentially duplicated Baker's performance this season and has a 4.47 ERA with a 108-to-24 strikeout-to-walk ratio through 149 career innings.

Glen Perkins: 4.14 ERA with 43 strikeouts and 19 walks in 78.1 innings

Glen Perkins was sort of a rotation afterthought after missing most of last year with a shoulder injury, beginning the season at Triple-A and joining the Twins as a starter in mid-May thanks to Baker's trip to the DL. He's stuck around in part because of Liriano's early struggles and in part because of his own solid performance, allowing more than three runs in just two of his 13 starts. Perkins is yet another extreme fly-ball pitcher, but hasn't missed bats as well as Baker or Slowey.

He also trails Baker and Slowey in terms of control, but the gap is relatively minimal and Perkins has made tremendous strides with his walk rate. After walking 10.3 percent of the batters he faced between Double-A and Triple-A, he's handed out a free pass just 5.5 percent of the time this season. A sample size of just 78.1 innings means that Perkins' control may not be as improved as it looks, but it wouldn't be surprising if pitching coach Rick Anderson has begun molding him into a strike-throwing machine.

Livan Hernandez: 5.44 ERA with 45 strikeouts and 23 walks in 120.2 innings

Livan Hernandez was signed in large part because the Twins were worried about going with a rotation consisting entirely of young arms, but he's been one of the league's worst pitchers while 26-and-under starters Baker, Blackburn, Slowey and Perkins have each found plenty of success. Prior to joining the Twins his ERA, WHIP, OPS against, and strikeout rate had all gotten worse in four straight seasons, so it should come as no surprise that Hernandez's steady decline is now in its fifth straight year.

Receiving the sixth-best run support in the AL has enabled Hernandez to hide his awful pitching behind a 9-6 record, but among the league's 46 starters who qualify for the ERA title he ranks either dead last or second-to-last in ERA, opponent's AVG, opponent's OBP, opponent's SLG, and opponent's OPS. He also has by far the worst Win Probability Added among AL pitchers at -1.72 and has been especially putrid recently, with a 6.86 ERA and .373 opponent's AVG over his last 11 starts.

Opponents have hit a laughable .342/.368/.506 in 540 plate appearances overall against Hernandez. To put that in some context, consider that Justin Morneau is hitting .323/.391/.512. In other words, he's more or less turned every hitter who's stepped to the plate against him into Morneau. And not only has Hernandez been absolutely awful, he's now keeping an annoyed Liriano from rejoining the rotation despite going 8-0 with a 2.53 ERA and 68-to-11 strikeout-to-walk ratio over his last 10 starts at Triple-A.

Joe Nathan: 1.13 ERA with 46 strikeouts and 10 walks in 39.2 innings

Ron Gardenhire's stubborn usage of Joe Nathan should be questioned given that he's on pace for a career-low 67 innings even after the bullpen lost Pat Neshek to a season-ending injury, but there's no doubting his dominance. After seeing his strikeout rate dip last season, Nathan has missed 12 percent more bats this year while handing out just eight non-intentional walks to 151 batters. He's converted 27-of-29 saves, leads AL relievers in WPA and ranks fourth in Expected Fielding Independent Pitching.

Opponents are hitting .191/.245/.277 versus Nathan, so the average batter has fared worse against him than Adam Everett (.189/.235/.324) prior to going on the DL. His sparkling 1.13 ERA can be traced to stranding 97.6 percent of his runners on base, which is both unsustainable and amazing given the AL average of 72 percent. Of course, his left-on-base rate was 83 percent from 2004-2007. Also of note is that Nathan has induced 49.5 percent ground balls after previously being an extreme fly-ball pitcher.

Matt Guerrier: 3.35 ERA with 38 strikeouts and 21 walks in 51 innings

Neshek's elbow giving out on May 31 pushed Matt Guerrier into the top setup role and he's responded with a 3.66 ERA and 16-to-6 strikeout-to-walk ratio in 19.2 innings since June 1. That's not quite up to Neshek's level and Guerrier hasn't been asked to put out nearly as many mid-inning fires, but he's kept his ERA under 3.50 for the fourth straight season while on pace for a second straight year logging over 85 innings. Claimed off waivers in November of 2003, Guerrier has a 3.22 ERA in 299 career innings.

Jesse Crain: 2.79 ERA with 29 strikeouts and 15 walks in 38.2 innings

Despite missing most of last year following season-ending shoulder surgery, Jesse Crain is throwing every bit as hard as he ever did, averaging 94.1 miles per hour with his fastball and 89.4 MPH with his slider. He predictably struggled some initially with a 12-to-10 strikeout-to-walk ratio through 20 innings, but has been fantastic since stepping into Guerrier's old seventh-inning role once the bullpen lost Neshek, posting a 1.45 ERA and 17-to-5 strikeout-to-walk ratio in 18.2 innings since June 1.

Dennys Reyes: 2.77 ERA with 15 strikeouts and 7 walks in 26 innings

You'd think that with Neshek injured and Juan Rincon released, Dennys Reyes would have taken on an expanded role in the bullpen, but instead he's faced a career-low 2.4 batters per appearance. However, his brief outings have typically involved high-leverage work, as Reyes ranks behind only Nathan (and Neshek) in Leverage Index while leading the bullpen with 38 inherited runners. Here's the inherited runner tally, which too often takes a backseat to ERA when evaluating relievers:

Reyes and Brian Bass have inherited by far the most runners with a combined 75. Meanwhile, Guerrier and Crain have combined to inherit 43 runners, and Nathan has entered a game with a grand total of two runners on base all year. While the logic behind bringing Reyes in to put out so many fires may be iffy, limiting him to such short outings makes plenty of sense. He's held lefties to .189/.232/.283 while righties have battered him to the tune of .325/.413/.500, and had a similarly extreme split last year.

Brian Bass: 5.31 ERA with 25 strikeouts and 19 walks in 57.2 innings

Bass is the bullpen's worst pitcher, yet along with inheriting nearly as many runners as Nathan, Crain and Guerrier combined, he leads the entire league in relief innings. Bass has gotten about 50 percent more work than Nathan and is on pace for 98 innings, although for the first three months at least most of that action came in mop-up situations. Recently he's been thrust into several high-leverage spots, leading to predictably ugly results on the way to allowing seven runs in his last four appearances.

There was little in Bass' extensive minor-league track record to suggest that he was capable of being a quality pitcher in the majors, yet he surprisingly cracked the Opening Day roster because he lacked minor-league options and the Twins were irrationally afraid of losing him on waivers. Meanwhile, Bass is a 26-year-old career minor leaguer who posted a 5.08 ERA in 359 innings between Double-A and Triple-A, so his current 5.31 mark is probably better than should have been expected.

Boof Bonser: 6.50 ERA with 61 strikeouts and 25 walks in 81.2 innings

Boof Bonser was banished from the rotation after going 2-6 with a 5.97 ERA in a dozen starts to begin this season; that made him 6-17 with a 6.02 ERA in 30 starts dating back to last year. On the surface his ugly 8.80 ERA in 11 outings as a mop-up man shows that he's been even worse since being demoted to the bullpen, but Bonser's increased velocity and 19-to-5 strikeout-to-walk in 15 innings as a reliever suggest that he may be capable of developing into a solid setup man if the Twins show patience.

Craig Breslow: 1.46 ERA with 13 strikeouts and 5 walks in 12.1 innings

Claimed off waivers from the Indians in late May, Craig Breslow has allowed just two runs in a dozen innings while stranding 9-of-10 inherited runners. He's been used almost exclusively in low-leverage spots and certainly isn't as good as he's looked thus far, but with a 3.24 ERA and 206-to-67 strikeout-to-walk ratio in 197 innings between Double-A and Triple-A, he has the makings of a decent middle reliever. With Reyes an impending free agent, Breslow could be auditioning for his job.