Arsenal boss Arsene Wenger was furious with referee Mike Riley's decision to give Manchester United a penalty in his side's away defeat.
Wenger insisted Wayne Rooney was not touched by Sol Campbell and claimed: "Riley decided the game, like we know he can do at Old Trafford. We were robbed.
"There was no contact at all for the penalty, even Rooney said so.
"It's very difficult to take to see how lightly the referee gives the penalty."
Wenger added: "We can only master our own performance and not the referee's performance.
"We got the usual penalty awarded against us when we come to Manchester United and they are in difficulty.
"It happened last season and it's happened again."
United's win means Arsenal's lead over Chelsea at the top of the table has been cut to two points, while United are now eight points behind the Premiership leaders.
But Wenger insisted United had not struck a psychological blow against his side in the race for the title after the loss of their 49-match unbeaten run.
He said: "I don't feel it will be a turning point. The season is long - they can improve and it can give them confidence but I feel we were a little bit robbed.
"It just shows you how hard it is to go 49 games unbeaten because sometimes you can lose games you don't deserve to lose.
"In football your confidence goes when your performances are not good but from this performance we can take away a lot of positives and I hope the players remember that.
"I think this defeat will make the players stronger because they feel a deep injustice."
Wenger was also unhappy with the lack of protection Riley offered Spanish winger Jose Antonio Reyes.
"At some stages there were incidents, especially on Reyes, where there was some deliberate kicking," stated Wenger.
"The rules are there to be respected and only the referee can make the players respect them."
Arsenal captain Patrick Vieira also criticised Riley's handling of the game.
"You get used to it when you play at Old Trafford, we are used to it," said Vieira referring to the penalty award.
"We'll be back and we'll be stronger. We're still eight points clear of United and two points clear of Chelsea.
"We're in a really good position and all the other teams would want to be in our position.
"People don't have to worry - we have lost only one game."