Warnock expects supporters to understand why England international Young opted to join the Red Devils.
He said: "I think Ashley will go down as a modern day great for Villa.
"I wouldn't expect anything less than a good reception for Ashley from our fans.
"He was a great servant to the club but it's not often the Premier League champions and Champions League runners-up want to sign you.
"We're a club who want to progress but Manchester United are at the top and Ashley has aspirations to be a top, top player.
"He has the ability to be one of the best in Europe and I don't think fans can begrudge him that."
Villa are hoping skipper Stiliyan Petrov will be back in contention to face United after he missed the goalless draw at Swansea with a hamstring injury.