You could ask the same thing of Girardi. He didn't seem like a very good manager, but strength-of-schedule and guys just coming around definitely helped Thomson. Was he partly responsible for the latter? Who knows.
It certainly doesn't feel like Mattingly has created a less complacent or more competitive evironment. He's pretty much playing the same cards. He just has an ace that Topper didn't. Even Sanchez had some bumps in April (a loss allowing four runs and that 12-hit, six-run outing the Phillies actually wound up winning).
