An excerpt from a pretty lengthy ESPN Insider article:

This is a big deal because Calderon, by all accounts, had been horrid on defense. He hurt his hip two games after the Atlanta disaster and has played only seven minutes over the past 13 games, a stretch that coincides with the Raptors' best defensive effort of the season.

As you might expect, very few Raptors fans consider this just a coincidence. As bad as Toronto has been on defense this year, it has been orders of magnitude worse with Calderon playing. The Raptors are 11.2 points per 100 possessions worse with Calderon on the floor than when he's off it -- roughly the difference between Golden State and the Celtics on defense. Subjective evaluation backs up the numbers -- he's been beaten off the dribble as easily as any other point guard I've seen this season, and that's been the case ever since he tore his hamstring early in the 2008-09 campaign.

Of course, Calderon's supporters will argue that part of the reason his on-court versus off-court numbers are so bad is that he has been out of the lineup when his teammates have finally started defending with some zeal. And it might be a fair point: As bad as he's looked, it's extremely unlikely that a single player could have an impact as profound as Calderon's on-court versus off-court numbers imply.

Of course, the Raptors have some recourse if it turns out Calderon was a major culprit. With Jack proving a reliable starter at the point and Weems capably filling in most of the minutes Jack left behind for the second unit, Toronto can plug Calderon into a reserve role, where his defensive shortcomings will be less glaring.

In other words, even if Calderon was the problem, he can't hurt Toronto as much going forward unless the Raptors use him the way they did pre-injury. Given recent developments, that seems unlikely.