Who cares? First of all, he had his best game against the Lakers. Second of all, its not like he's going to start playing like shit once he starts playing teams above .500. Who on the Heat guards him? Chalmers? No. Even if he has trouble against Rose, Westbrook, and Rondo, it really doesn't matter. He's basically a rookie, and he's doing what he is supposed to do: he is playing well against, and beating the teams he is supposed to be beating.
The real test will be when Carmelo comes back. No way he can keep the pace he's at with Carmelo dominating the ball. Will they be able to co-exist? Carmelo is probably the toughest player to play alongside in the league (he doesn't create for anyone, and he's not that good off the ball). D'Antoni would be wise to stop allowing Carmelo to play point-forward, and have him play more of a traditional 3.