Maybe the fact he just came back from a major surgery and you don't want to over stress that knee....or that he's not the same player and probably shouldn't quite play unless they're showcasing him.
He can't keep up with the team to finish fast breaks on a fast breaking team. Then he calls for the ball and isolates in what is suppose to be a motion offense. BUt with Ariza playing like garbage, I see more fans and RA looking to McGrady for a few more minutes.
Because Adelman likes how his rotation is playing, and he doesn't want to penalize an existing player (Ariza, Battier, Budinger) just to add McGrady. So right now the Rockets just want to see how McGrady looks for a few games, and after that they'll decide on how to ramp up his minutes.