I'm sure Marcus can improve some, but things don't look good.
He showed all of the same flaws in college. He decides what he's doing early, and doesn't adjust well. His balance is poor. His shot has a huge amount of motion to it, and he has always taken too many bad shots. He's only an average passer. He can't finish with his left hand at all. The only progress I've seen is he has slowed down his dribble penetration a little, so he is a bit more under control. But even that hasn't translated to improvement, because that gets him into the teeth of the defense, and he isn't particularly clever when he gets there. He's an athlete, and a tough guy, but he's never shown much offensive promise.
You can usually tell when a player can improve. RJ Hunter is a good example. His decision making is consistently both good and quick. His balance is good. He takes shots you want him to take. He's shooting poorly, but that is a skill that can improve with practice and experience.