On every draft website they talk about him having past knee issues. Its impossible to know the extent of them though since he is only 18/19 and hasn't played competitively in a while. I'm sure once workouts get going and teams medical staffs are allowed to examine players we will all know more about it.
On every draft site? Like what, NBAdarft.net? More than half of the material you find on the net is regurgitated from there, and that site ****ing blows. The ONLY thing I've found is he had tendinitis in one of his knees more than 2.5 years ago. Maybe he has legitimate knee problems, but with the actual information out there (almost zero substantiated evidence), that's hardly fair. I'd bet around half of the players in any given draft have had an ailing knee at some point. Having an ailment a couple of years ago isn't sufficient to say a guy has bum knees.
And on the best website accessible to the public that there is and logging the largest tracking of Kanter that there is (at least in english) - draftexpress - it doesn't mention anything about a knee problem. Not even once.
Unless someone wants to enlighten us with something that is not accessible to the public and is substantiated. Either way, we'll find out what's going on in his legs when he has to take a physical soon enough.