Alex Ferguson has called for the Football Association to launch an investigation into the "terrible" way that a posse of Arsenal players harassed Ruud van Nistelrooy at Old Trafford.
Van Nistelrooy had angered the Arsenal contingent for his part in Patrick Vieira's red card with 10 minutes left.
When the Dutchman then missed a last-gasp penalty, he was surrounded by Arsenal players celebrating right in his face, with Martin Keown appearing to push him at one point.
When the match ended soon afterwards, van Nistelrooy was again converged upon by several angry opponents as tempers boiled over even further.
The FA will have to wait for the reports from the referee, Steve Bennett, and the fourth official, Neale Barry, but they are almost certain to instigate an investigation.
Ferguson declared: "I really think that was terrible behaviour at the end. I think the FA will look into it.
"Great credit to Ruud for the way he came off the pitch but you don't want to see that kind of behaviour. I think it is beyond the pale.
"Everyone in the country will be disappointed to see that kind of behaviour. After all, these are the best two teams in the country," he said.
"Over the past few seasons, Arsenal and Manchester United have been the vanguard of our country, so you would expect better."
However, Arsenal boss Arsene Wenger insisted: "If you want to convince me that Martin Keown is a devil and van Nistelrooy is an angel then you will have a big problem.
"Van Nistelrooy is quite provocative and I would say that every Arsenal player would confirm that," he said.
"Of course, you cannot be unhappy when your opponent doesn't score a goal, but their attitude was not linked to the penalty. It was more linked to his overall attitude during the game."