Nah, I actually meant "at worst." At worst, he plays ineffective defense (as he doesn't contest shots with his hands up as often as he should), but that may have very well been due to the Wolves' defensive ideology of "don't foul," which is why they had such a low foul rate as a team. All in all, his overall defense doesn't hurt his team and in some cases, actually helps.
His help defense is actually considered the weakest aspect of his defense (largely because he's usually out of position while focusing on nabbing potential rebounds), with his man-to-man post defense considered pretty good. According to Synergy, he's 43rd in the league at points given up per possession (.72 vs David Lee's .88, ranking him 156th, for comparison). The team's defensive rating with him on the court was actually better than with him off the court this past season.