Three United player stood in offside positions as the free-kick was about to be taken, then all ran forward at the last moment, distracting keeper Jayson Leutwiler.
"Midtjylland - our Europa League opponents - used to do that, so we asked the ref if we could and he said yes. So we did it and it worked out very good," said a very pleased Mata.
Reds boss Louis van Gaal, however, didn't seem as impressed. "It is unbelievable because I think the rules say that you cannot disturb the view of the goalkeeper," the Dutchman admitted.