Watford manager Marco Silva claimed Marcos Rojo should have been sent off for a second yellow card when he mistimed a tackle on Roberto Pereyra in the United penalty box with seven minutes plus stoppage time left in the match.
The Portuguese gaffer told Sky Sports: "We scored the first goal from a penalty and for me in that moment it was a second yellow card for Marcos Rojo, with 11 against 10 it would be really difficult for them.
"We made it difficult for them because we scored the second goal. Imagine if it was 11 v 10 at that moment. We started to create more problems for them, but then one counter attack from them and we should do different in that moment."
The former Hull City manager felt Tom Cleverley could've done more to stop Manchester United's fourth goal, which effectively killed the game:
"Tom went to stop the counter attack, at that moment we should do a tactical foul to stop Lingard because the game would continue to be open until the end if they didn't score the fourth."
Silva also pointed to nerves as the reason for his side's poor start to the match, which saw them concede three goals in the opening half an hour of the fixture:
"After the first goal I felt our players were a little bit nervous, the third goal is an example - we had two or three times to deal with the problem and we made easy mistakes."
Finally, the Watford manager revealed he'd had words with his players and that he was much more impressed with their second-half display: "It was different in the second half, I told my players it wasn't in our image to make lots of mistakes like we did after the first goal."