Wes Brown is convinced he was fouled just as John Terry rose to head the only goal of the game for Chelsea at Stamford Bridge on Sunday.
Manchester United came away from West London with a strong sense of injustice after a confident display went unrewarded against the Blues.
Red Devils manager Sir Alex Ferguson has vented his fury at referee Martin Atkinson for failing to spot Didier Drogba clearly pulling at Brown's arm as Terry headed home.
The champions have also questioned whether the free-kick which Chelsea scored from should have been awarded after television replays clearly showed that Darren Fletcher had won the ball from Ashley Cole.
"It is very hard to take," he said.
"Drogba was holding me and he pulled me down. He has obstructed me from trying to jump for the ball. But the referee did not see it. What can you do?
"I didn't think it was a free-kick in the first place either. Darren has not gone in viciously.
"It looks like he has just turned, and Ashley Cole has jumped in the air. We definitely did not deserve that result. We were the better team. We passed the ball well and had more chances.
"It is disappointing to be five points behind them now. We just have to try and put this result behind us, carry on playing like that - but just score goals."
Fellow defender John O'Shea is confident the result can galvanise the squad and help them go on a lengthy unbeaten run to help overhaul Chelsea at the top.
"We hate losing games and we know that we now have to go on a great run," he told MUTV.
"The international break comes at a bit of a bad time - you'd prefer to have another game a few days later to help put the defeat out of your mind.
"There's a bit of a sour taste left, but hopefully we'll get over that. Not many teams come to Stamford Bridge and dominate games.
"We had great spells where we had Chelsea really pinned back and we caused them problems on the counter attack as well, but our final ball or shot just didn't go for us and we were unlucky at times as well.
"We stuck to our game plan very well and were the better team."