Allardyce felt his side should have been awarded a spot-kick last night as they crashed out of the FA Cup with a 1-0 defeat against United.
Wayne Rooney scored the only goal of the third-round replay in the early stages, but also missed a second-half penalty after Jordan Spence was adjudged to have handled the ball by referee Phil Dowd.
However, Allardyce was furious West Ham had not been given a spot-kick of their own for Rafael's apparent handball shortly after the interval.
"There's no doubt about the difference between Rafael's handball and Jordan Spence's," he said. "Spence plays for West Ham and the away team, while Rafael plays for the home side at Old Trafford.
"With Rafael, the ball hits his hand but no penalty is given. With Jordan Spence, the ball hits his hand and it's a penalty.