He's either got an ankle or knee injury. Apparently it's more serious than they thought since they're just now putting him on the injured list. Either way I expect he'll be back by playoff time or at latest, if he decides to give it as much time as possible and the Mavs are a #1 seed, by the second round.
Actually it's a hamstring injury on his right leg. He pulled it in the game against the Kings on March 16th and appeared to be getting better however in a practice before the T-Wolves game on March 30th he tweeked it again and was placed in the IL. He'll more than likely sit out the rest of the regular season but is expected to be ready for the playoffs (at least according to Nellie).