Louis van Gaal believes a "very bad" Manchester United defeat to West Brom could have been avoided had referee Mike Dean used his discretion and not sent off Juan Mata.
United's recent resurgence was halted as a crucial week got off to a bad start at The Hawthorns, where the Spaniard's first-ever red card was key to the visitors' downfall.
Mata picked up two bookings in less than three first-half minutes and Albion capitalised after the break, with Salomon Rondon capping an impressive display with a fine strike that secured Tony Pulis' men a 1-0 win.
The result ended United's four-match winning run and is not the ideal preparation for a week that sees a Europa League trip to Liverpool followed by an FA Cup quarter-final against West Ham.
"I think always this match has an influence on the next match so it is, for us, very bad," Van Gaal said. "It is a bad moment and it wasn't in my opinion a necessary loss.
"I don't want to say that we deserved to win, no, but we don't deserve to lose.
"When you see at the end the chances that West Bromwich Albion has created in comparison with us, it was an unnecessary loss.
"When it is a draw and you play 60 minutes with 10 men, you can accept it - but now you lose and you lose contact with the first four or five."
United end the weekend sixth in the Barclays Premier League, three points behind fourth-placed Manchester City.
Manuel Pellegrini's men also have a game in hand and Van Gaal believes things would have been different had Mata not been sent off.
The Dutchman told broadcasters the first booking was "stupid" but suggested referee Dean should have taken the Spaniard's character into consideration before brandishing a quick-fire second.
"My opinion doesn't count," Van Gaal said when asked about Mata's red card.
"You give as a referee within five minutes two yellow cards to the same person and you know also who is Juan Mata.
"I asked him a rhetorical question: 'How many red cards (have you had in your career)?' None (he said).
"I had assumed that already so, yeah, as a player you have to know also when you have a yellow card that you have to take care of that."
Van Gaal was then asked if referees should use more discretion and, having initially misunderstood, said with a laugh: "I think that all the referees already are doing that. Normally they are doing that."
The Dutchman called Mata's red card the "turning point" in a match that was settled by Rondon's fourth goal in six Premier League matches.
It was Albion's first league win at home to United since March 1984 and, more importantly, almost certainly secures their top-flight status.
The Baggies now sit on 39 points and, having jokingly dismissed captain Darren Fletcher's target of 50 as "nonsense", Pulis is focused on reaching the 40-point mark.
"Let's get to 40 and then take it from there," the West Brom head coach said. "I think 40 points is always (required).
"It is very difficult managing teams in the lower half of the Premiership. It is really, really hard work mentally and physically.
"You take it on board and worry about every result, your own and everything else.
"It can drive you insane at times and this is exactly the same as the clubs I have managed. You never stop worrying, never stop thinking that things can turn.
"You have to keep driving people and driving people through. I think we deserved it in the end."