Erik ten Hag slammed the use of VAR in the Premier League as "inconsistent" after Casemiro was sent off for a second time in his Manchester United career.
The midfielder was shown a red card for a high challenge on Carlos Alcaraz during the 0-0 draw with Southampton on Sunday after referee Anthony Taylor was advised to consult the pitchside monitor by VAR Andre Marriner.
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Ten Hag was incensed by the decision, particularly after Leicester City defender Ricardo Pereira escaped punishment for a similar challenge on Chelsea's Joao Felix on Saturday.
Ten Hag also thought United should have had a penalty when Armel Bella-Kotchap appeared to handle Marcus Rashford's cross in the penalty area.
"What I think is the inconsistency, players don't know anymore what is the policy," Ten Hag said. "I think, all across, you see this weekend Leicester versus Chelsea, VAR is not coming on the line [the pitchside monitor].
"Today, it is coming on the line, and today two penalty situations, they don't come on the line. I think especially the first, it is clear and obvious handball. So, what is the policy?"
Casemiro has now been sent off in two of his past three Premier League appearances after being shown a red card for an altercation with Crystal Palace midfielder Will Hughes in February.
The Brazilian is set to miss United's next four domestic games, but in his postmatch news conference, Ten Hag was keen to defend the 31-year-old former Real Madrid star.
"There is another one inconsistent, the refereeing is coming at the start of the season with a policy: 'We are [the] Premier League; we are going strong and want intensity,'" Ten Hag added. "Casemiro is across European leagues in over 500 games and he had never a [straight] red card, and now he has twice. He plays tough, but he plays fair. Also in this he is playing fair. Also against Crystal Palace. It is very debatable.
"Everyone who knows something about football -- when you freeze, it looks bad -- but everyone who knows something about football, they know what is bad and what is fair. Casemiro is a really fair player, tough but fair. It shows, over 500 games in big leagues, never sent off."