Yeah he's not going to be drafted, but the sleeper concern here with European kids is getting offered a decent chunk of change to play professionally at home. Filip Petrusev and Omer Yurtseven being a couple of the big names from last year off the top of my head. I think he probably returns but I don't think NBA prospects are the real threat here.
As for next year, maybe he has a shot but he's going to have to come in and really knock down jump shots. I think mechanically there's nothing majorly broken but at 6'7" he's going to have to be a wing in the NBA. The guy in this class that he reminds me of is Kessler Edwards at Pepperdine who I think will go in the 2nd round. They're similar profiles in age, size, college level of competition, and on-ball defensive acumen, but Edwards shot 49.1/37.8/87.6 and Mballa shot 48.6/25.0/67.9. Mballa will have to make a huge jump there to have a chance to be drafted