Edwin Jackson wouldn't like to pitch in the bullpen, but if it's necessary, he'll do it.
“At the end of the day, it wouldn’t necessarily be about me. It would be about the team. I would like to stay starting — I’m comfortable starting — but if I have to go to the pen, I’d just go down and deal with it.’’ -- Chicago Sun-Times
While the Sox still shouldn't get away from the six-man rotation just yet, if they do, Jackson will probably be the guy to be moved to the bullpen. As Daryl Van Schouwen points out, Jackson has previous relief experience and throws right-handed, thus making him a more attractive option than John Danks—even if Danks has pitched the worst out of any starter since the six-man rotation was implemented.
Jackson has been an interesting case this season, as he sports a 4.63 ERA but a sparkling 3.26 FIP. While his strikeout and walk numbers are right about in line with 2010 (the full season, not just with the White Sox), he's done a fantastic job limiting home runs.
What's hurting Jackson is opponents' .352 BABIP against him, which usually would signal some bad luck. Maybe it does. But 23.7 percent of batted balls against Jackson have been line drives, which is usually a pretty good indicator that a pitcher has been hittable. Jackson's tERA (explained here) is about average, which tells us Jackson has been roughly average in generating outs on balls in play. Conversely, on the three true outcomes (walks, strikeouts, home runs), Jackson has been great.
It's certainly a little odd of a mix for Jackson, who alternates between looking unhittable and hittable from start to start, depending on his slider. He's also been fairly inefficient with his pitches—in his May 22 start against Los Angeles, he cruised to strike out seven and allow one run with 10 ground balls...in 5.2 innings.
That being said, with four months remaining in the season, I'll bank on Jackson's ERA regressing down into the threes at some point. That is, of course, if he remains a starter.
Also, what bullpen role would whatever starter gets dumped fill? Would it really be worth it for Jackson to work as a better version of Tony Pena? The low-leverage innings I fear the starter-turned-reliever will get will mostly be a waste, especially if the Sox send a guy making $8.35 million [who took a top-flight pitching prospect to acquire] to pick them up.
So, once again, the best option for the White Sox is to stick with the six-man rotation.