The Belgian believes if he can mark Rooney out of their Champions League clash, Bayern will beat United.
Van Buyten, who had a spell at Manchester City, said: "Rooney is a big danger as one of the best players in the world, but it's not all about him because United have other great players.
"But Rooney is spectacular. We know if we stop him then United will not be so dangerous. That will be the key.
"However, even though we may be without Arjen Robben, I wouldn't say United are favourites. I would call it 50-50.
"I think that if we win this game, and the tie, then it will show everyone that Bayern Munich are back in Europe and in the world as a major force."