Marco Silva felt Manchester United should have been a man down before scoring their killer fourth goal against Watford on Tuesday.
Ashley Young had opened the scoring on 19 minutes at Vicarage Road before adding a second on 25 minutes.
Marcos Rojo was yellow carded on 38 minutes and was at fault as he conceded a 77th minute penalty with a foul on Roberto Pereyra that started the Hornets' come-back as Troy Deeney scored from the mark.
Abdoulaye Doucoure added a second for the hosts on 84 minutes, but Jesse Lingard made sure of the points with goal on 86 minutes – for a 4-2 final score.
But Silva is adamant that Rojo should have been sent off.
After the game, a disappointed Silva said: "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.
"Tom (Cleverley) went to stop the counter attack, at that moment we should do a tactical foul to stop [Jesse] Lingard because the game would continue to be open until the end if they didn't score the fourth."
He added: "It was really hard for us [after going 3-0 down]. Nobody expected what happened in that moment but against this type of teams we knew before the match they are very strong on the counter attack."