The Hammers boss believes that Robin van Persie's equaliser 13 minutes from time should have been ruled out for offside.
"We had a fabulous win taken away from us by an assistant referee with Robin van Persie's equaliser," BBC Sport quotes him as saying. "It was blatantly offside as we can all see.
"We would have been satisfied with a draw before the start but when you see a victory against one of the best teams in Europe taken away from you it is hard to take."
The draw leaves United 13 points clear of local rivals Manchester City at the top of the table, while West Ham move about Southampton into 11th.