Considering his walk rate almost doubled with the Cubs, his strikeout rate fell by about half, and his BABIP against went from .367 with the Sox to .225 with the Cubs, I am going to note that your assumption was wrong that he was able to "locate the plate". His FIP with the Cubs was actually 3/4 of a run higher with them vs the Sox, and note that he was incredibly lucky with the Cubs, and incredibly UNlucky with the Sox. I will also note that he had two saves in 25 appearances, so he wasn't exactly "closing out games" either.