Manchester United manager Jose Mourinho was unhappy with Michael Oliver's decision to wave play on when Watford took the lead in the first half.
The Portuguese believes Miguel Britos fouled Anthony Martial and his side should have been awarded a free kick.
He says he has "no control" over such decisions as he urged his players to improve ahead of next week's home game against Leicester City.
"I'm always concerned when we don't get the result we want," he told reporters.
"That's an obvious situation [a foul by Miguel Britos in the build-up to Watford's opening goal].
"I think there is no debate, no controversy. I think that is an obvious situation, like it was obvious last week against Man City but it's something I don't control and I cannot even be critical of.
"In the eyes of everyone, it doesn't matter whether you're a Manchester United fan or a Watford fan or whatever, it was obvious.
"I think in the first half, for about 25 to 30 minutes, we didn't play well and that is something we can control, we can improve and we can work on it.
"The referee and linesman's mistake is not under my control.
"I can't do anything to improve it and, in the second half, we were much better. We got the equaliser and, after that, we were the team [pushing for a winner]. [Heurelho] Gomes makes a phenomenal save and the luck is against us, something we cannot control.
"Collectively and individually, we need to get better."