Sir Alex Ferguson complained that Manchester City's opening goal should have been disallowed after Roberto Mancini's team beat the league leaders 2-1 at Old Trafford.
City substitute Sergio Aguero scored the deciding goal but Manchester United manager Ferguson felt referee Mike Dean was wrong to allow James Milner's opener.
He insisted City striker Carlos Tevez was in an offside position and obstructing goalkeeper David de Gea's vision.
He said: "The first goal, certainly Tevez is interfering with play. But the linesman on the other side? There is no way he was giving that. It was disappointing as he was clearly in the road of the goalkeeper. If that is not interference then I don't know what is."
Ferguson was unhappy with his side's defending for both City goals, adding: "In the second half we were the better team. We have to look at ourselves in terms of the goals we lost, we know Aguero is a fantastic finisher, that is what they brought him on for. We gave him a lot of room for the goal."
However, the United manager was pleased with the performances of his strikers and Phil Jones, who moved into defence with Nemanja Vidic, Jonny Evans and Chris Smalling all absent with injuries.
He added: "There was a lot of plus sides and evidence our form is good. Robin van Persie was fantastic, almost 90 minutes for Wayne Rooney will bring him on again after missing a couple of games.
"Phil Jones was magnificent. He's only played two games at centre-back this season and that tells you what a good player he is, at 21 years of age he is a man. He was fantastic."
United could be without the injured Ashley Young when they travel to Stoke on Sunday but Ferguson added: "The more important thing is trying to get one of the centre-backs ready. That will be important."