Let's sum it up:
If Suppan remains in the AL Central, he'd face the Sox about five times or so. Let's say he wins 4 of those games.
If Suppan joins the Sox, he starts 33 games. Let's say, he loses 15 games. I'm willing to bet that he blows more than 4 games out of his 15 losses. By your methodd, we'd actually WORSEN than improve.