The Midwest League can certainly be a turning point in a minor leaguer's career. Some short-season studs flail in the league, while others really take off, zooming up the prospect map.
Anderson and Cahill both entered the Midwest League (Cahill for Oakland affiliate Kane County and Anderson for Arizona affiliate South Bend) with reputations as strike-throwers with low-90's two-seamers and decent offspeed stuff. Both were second-round draft picks in 2006, and thought to be potential third or fourth starters in the majors.
By the end of the 2007 season, Anderson had been promoted to the California League, and both he and Cahill were considered potential front-of-the-rotation pitchers. Anderson had a 2.21 ERA in the Midwest League that year, and an even better 2.03 FIP, thanks to an 85/10 K/BB and just three homers allowed in 81 innings.
Cahill, on the other hand, spent the whole year in the league, throwing 105.1 innings. He also allowed three homers. Cahill showed a bit more of a strikeout tendency than Anderson (117 K), but worse command (40 BB). His ERA was 2.73 and his FIP 2.74.
Both pitchers were 19 at the time, so it was way advanced for their age to be carving up full-season ball like they did. Anderson's staggering command, Cahill's excellent strikeout numbers, and the groundball tendencies of both made them consensus top prospects, and they have largely delivered on their promise.
Fast forward three years, and Anderson and Cahill are Oakland mainstays, while another 19-year-old considered a decent-but-not-overwhelming prospect is carving up the Midwest League. This time around, it's lefty Ian Krol, who's actually a few months younger than Anderson and Cahill were when they blazed through the circuit in 2007. How does Krol's 2010 stack up to Anderson and Cahill's stellar work in the Midwest League?
Krol entered 2010 with the same reputation that Anderson had entering 2007: he's very polished for his age, and has advanced offspeed stuff, but his fastball isn't special enough to make him an ace. The first thing that jumps out about Krol's 2010 season is his 1.98 ERA, which would get him noticed even if he were four years older. He's allowed just 16 earned runs (22 total runs) in 72.2 innings this year for the Cougars.
A good way to keep runs down is to keep the ball in the park, and Krol's done just that. He's only allowed one home run all season, a solo shot to Bowling Green second baseman Tyler Bortnick on June 4. Krol also has an Anderson-esque 14 walks in those 72.2 innings. He appears to be keeping the ball in the zone and out of hitters' wheelhouses by not giving up many walks or homers.
A cautionary note on the homers, however: Krol only has a groundball rate of 45%, which is merely average, and not what one would expect from someone with such a low homer rate. He has a 58% groundball rate, but only 39.2% to righties. The Midwest League has a pitcher-friendly reputation, particularly in the early-season cold weather, so it's very possible that quite a bit of Krol's lack of homers allowed is due to a combination of luck and environment. Still, it's safe to assume he won't end up with a homer problem in the majors, especially in Oakland.
Krol also has only 55 strikeouts this season, good for a good-not-great 6.81 K/9. Both Anderson (9.41) and Cahill (10.00) struck out over a batter per inning in their 2007 campaigns, so the strikeouts are the one area where Krol's season stands out as inferior.
It appears that a good amount of Krol's sub-2.00 ERA comes from the homer luck and a .254 batting average on balls in play, which isn't likely to sustain itself at that level. His FIP is 2.97, about a run higher than his ERA, and if you correct some for the homers, you get a pitcher deserving of an ERA in the low-to-mid-3's.
That's not to take away from Krol, though: Any 19-year-old who can succeed in full-season ball could well have a bright future, and he's shown extremely impressive command for his age. As for his ultimate projection, it's difficult to throw him in the same boat Anderson and Cahill were in post-2007, largely because of the strikeout rate.
Krol's K rate is already below 7 K/9, and like many pitchers, he's likely to shed some strikeouts as he moves up and faces better hitters. In the majors, the highest-ranked xFIP for starting pitchers with under 7 K/9 in 2010 is that of the Angels' Joel Pineiro, at 3.88. That's 26th in the majors. The only pitcher in the top 10 of xFIP rankings with a K/9 under 8 is Roy Halladay, at 7.75, and he's there largely because of his miniscule walk rate.
In order for Krol to get into that upper tier of pitchers, he's going to have to find a way to miss more bats while retaining his lack of walks, which is an extremely difficult thing to do. It's possible, given how wacky career paths can be, but scouts don't talk about Krol as a projectable pitcher very often (Then again, they didn't say that about Anderson either, and he picked up 3 MPH in the majors on his fastball (and nearly 5 MPH from that season in the Midwest League, so who knows?).
Ultimately, Krol's likely role will be similar to Dallas Braden's: a lefty who simply refuses to walk batters and does everything well enough that he's a solid mid-rotation pitcher. It's not as flashy as Anderson or Cahill, but Krol certainly could be a significant contributor in Oakland.