David de Gea appealed for "more consistency" from referees after Manchester United had Casemiro sent off in their 0-0 draw with Southampton.
United failed to get back to winning ways in the Premier League following their 7-0 thrashing by Liverpool last week.
They spent much of a home game with the division's bottom club on the back foot, especially after Casemiro became the first United player to receive two red cards in the same league season since Nemanja Vidic in 2013-14.
Casemiro was initially booked by Anthony Taylor for his challenge on Carlos Alcaraz, but the referee was instructed by VAR to consult the pitch-side monitor.
Having looked at replays that showed Casemiro going over the top of the ball and sending his studs high into Alcaraz's leg, Taylor changed his decision and showed the Brazil midfielder a red.
United claimed a point despite Southampton attempting 17 shots, with De Gea called into action to preserve their clean sheet on several occasions.
A point for 10-man United.#MUFC || #MUNSOU— Manchester United (@ManUtd) March 12, 2023
Speaking to Sky Sports, De Gea said: "It should be a game to win today, but the game changed when we lost a man in the team.
"We tried our best to score, but it was difficult. They had some chances, but in the end we showed good team spirit. We have to take the point and keep going.
"I think Casemiro was unlucky. He tried to touch the ball and his foot came up high. I think the referees need to show more consistency. Sometimes they show a red card, and sometimes they don't."
Casemiro is now set to miss the next four domestic games for United. He will be unavailable for the FA Cup quarter-final with Fulham as well as Premier League clashes with Newcastle United, Brentford and Everton.
"It's going to be tough, he's a big player," De Gea said. "We will miss him for four games, but we have a big squad. We have players coming from the bench who do well. We will keep working hard."