The Premier League meeting had been going on for just over a quarter of an hour when the situation occurred.
In connection with a corner, David de Gea happened to be stamped by teammate Fred and lay down on the grass in great pain.
The ball bounced off to Emile Smith-Rowe who could easily shoot in 1-0.
First, a free kick was awarded by referee Martin Atkinson, but after a video review, the decision was reversed and the case was approved.
The Manchester United players protested wildly but were not heard.
Just before the end of the first half, however, United would equalize through Bruno Fernandes.
The match is in progress.