But he has faith in centre-backs Dunne and Collins who have played major roles in Villa having the best defensive record in the Premier League this season.
Warnock said: "Rooney is one of those players I love watching on TV. You would put him, Messi and Ronaldo in the same bracket as the three best in the world.
"You could argue all night about who is the best out of those three but he is one of those players you want to watch which is something that can't be said about every player.
"He has got all the ability in the world. I've been lucky to train with him (with England) and realise how good he is and it is even better when you train with him. You realise what he can do.
"Richard and James have got a tough task ahead of them - but Rooney will find it tough to do much against those two as well.
"They are two of the best defenders in the league on form and have been fantastic all season."
Warnock believes Villa have the attacking capabilities to pose United with as many questions attacking-wise as they will face from Rooney.
He said: "We are blessed with good players on both wings and centre-forwards and midfielders.
"They are all inter-changeable all of the time and make it difficult for defenders to mark them.
"I am sure United will be looking at our team and wondering who they are going to try and stop first.
"Then they will have to look at the next one, and he is as good as the first one, and that's where we are lucky.
"We have players who can cause teams problems all over the pitch."
Man. Utd 5/6, Draw 12/5, Aston Villa 7/2