Anyone can see that Carter's explosiveness (particularly in the vertical direction) is substantially diminished this year. As for whether that's because of his knees, his still-ailing ankle (remember RJ last year?), or something else, that's pure speculation. Magic Johnson is hardly a guy who has any personal experience with the kind of athletic ability Carter was blessed with, so he's undoubtedly purveying naked assumptions based on what guys like James Worthy felt when their athleticism started to decline. And, of course, he was making the case as sensationally as possible to entertain a national TV audience.
Even if Johnson is correct, however, and Carter's loss of leaping ability is due to some chronic degeneration in the knees, he's selling Vince WAY too short to say he's "just another player". Show me "just another player" who can finish the kinds of drives he did last night (or in Sacramento) or make the kinds of drive and dish plays he did (if Boki and Boone could finish AT ALL with a defender in the same zip code, Vince would have had another 4 assists easily). If "just another player" can get you 29/6/4 on only 20 shots (with several Sportcenter hilights thrown in for icing), then I'll take 12 of those in a heartbeat.
Carter's overwhelming athleticism has always blinded certain people to how versatile and fundamentally sound he is offensively. He may not go over people anymore, but the last two games alone show that, when he maintains an aggressive mindset (which farouq710 nailed as the REAL key), he can still go around and under people. Yes, he always dazzled me with what he could do in the air. But as someone who's watched every televised game he's ever played in the NBA, I've always been equally amazed at the creativity, variety of releases, and touch he exhibits in finishing other kinds of plays around the basket. Put that together with the fact that his midrange jumper is probably better this year than at any other time in his career (save early in the season when he was suffering from the injury to his shooting hand), and you are still talking about a guy who, with proper emphasis in the schemes, is a top 10-15 scorer in this league and one who also brings far better playmaking skills than most others in that category. He is also still one of the best rebounding guards in the league.
What I sense has been happening with Carter this year, and what partly accounts for the ups and downs in his style of play, is that he is still adjusting mentally to the new limitations of his body. He is clearly making a better effort than ever before not to "force" his offense and has curbed his shot selection considerably, to the detriment of the team, in some cases. (Of course some in these parts claimed to have wanted only that from him for several years.) But the reality is that he still needs to be the number one offensive option for this team to look competent offensively, and he NEEDS to shoot more than he has been. I would love to see more quick hitting, catch and shoot curls run for him, because I think the midrange part of his game is the part that most needs to be expanded and exploited at this point in his career.
Of course none of this is going to make any difference in this team's success unless they acquire at least one legit shooter and learn how to defend again as a team.