Erik ten Hag criticised "inconsistent" officials after Casemiro's second red card this season forced Manchester United to settle for a 0-0 draw against struggling Southampton on Sunday.
Casemiro was sent off in the 34th minute at Old Trafford after initially being booked for his high challenge on Southampton's Carlos Alcaraz.
VAR official Andre Marriner advised Anthony Taylor to review the Brazil midfielder's foul on the pitchside monitor and the referee upgrading the offence to a red card.
Having bounced back from their 7-0 humiliation at Liverpool last weekend by beating Real Betis in the Europa League on Thursday, United had to dig deep to avoid a shock defeat against bottom of the table Southampton.
United manager ten Hag was furious with the decision to send off Casemiro, who was also dismissed against Crystal Palace in February, and complained about penalty calls that didn't go his team's way.
"Well, what I think is the inconsistency (means) players don't know any more what is the policy," Ten Hag said.
"I think it's all across. I mean, you see yesterday, Leicester-Chelsea, then VAR is not coming on the line.
"Today it's coming on the line and then it's two penalty situations, but they don't come on the line.
"I think especially the first one was for me clear and obvious handball, so what is the policy?"
Ten Hag said he had "some questions" about the officiating, with the influential Casemiro now facing a damaging four-match suspension as United look to wrap up their place in the top four.
"There is another one inconsistent," Ten Hag said. "The refereeing is coming at the start of the season with a policy 'we are Premier League, we are going strong here, we want intensity'.
"Casemiro is across European leagues, in over 500 games he had never a red card and now he has twice.
"He plays tough but he plays fair, also in this he is playing fair. Same as against Crystal Palace, so it is very debatable.
"When you freeze it looks bad, but everyone who knows something about football, who is acting on top football, they know what is bad and what isn't bad, what is fair.
"I tell you Casemiro is a really fair player. Tough but fair and it shows over 500 games in big leagues, never sent off."
Ten Hag replied "we will see" when asked if United will appeal a decision that left United playing the best part of an hour with 10 men.
"Once again this team showed big spirit, team spirit, good fighting spirit, supporting each other. We played I think with 11 and with 10 a really good level," he said of the stalemate.