Sumner's season ended in January when he tore his left ACL. Before the injury, he was projected as a first-round pick. ACL tears for guards aren't seen as big-time red flags anymore, not the way they were even a decade ago. But Sumner has not been able to work out. Without teams getting to see his physical capabilities so close to the draft, he's essentially losing millions of dollars and a guaranteed contract.
He's got a really good first step, can rise high when he shoots, knows how to use angles and is a nice combo-guard blend. In the NBA he'll be asked to be a pass-first point guard, but that's not exactly his game, and the NBA is drifting away from this mold as is. Sumner will succeed if he stays healthy and improves his shot, which I think is inevitable.
Sumner has lost a season and a half's worth of college games due to injuries, the first being tendinitis as a freshman, the second being an ACL tear as a redshirt sophomore. There's also a potential (prior) problem with a labrum. Putting aside the injuries, other concerns linger about Sumner's conditioning and how long he'll take to get into NBA shape, plus a potential loss of his grade-A athleticism that made him a prospect in the first place.