He accused referee Kevin Friend of costing Everton a point because of a refereeing blunder at Old Trafford.
Martinez believes Radamel Falcao's goal should not have been allowed to stand though as he felt the game should have been stopped to allow midfielder Steven Pienaar to receive treatment for an injury he sustained moments before Falcao found the net.
"We're disappointed with the second goal. I thought the referee made a major, major error," the Everton manager said after the 2-1 defeat.
"I just don't understand that decision.
"Steven Pienaar was on the ground, we made it clear to the referee to stop the game and he allowed it to play on and it was very, very disappointing.
"Steven receives a knock and he goes to ground, the ball is in front of him and the referee is well aware of it. He is talking with fourth official.
"It is a clear-cut decision. The law says if the player is on the ground and the ball is not in a dangerous position you need to stop the game.
"Man United attacks and the ball ends up in our net and I don't think that is fair. I think that action was a big mistake from the referee."
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