Having bounced back from the seven-goal shellacking at Liverpool with a 4-1 win against Real Betis in the Europa League, the Red Devils had to show their character again.
Casemiro was sent off in the 34th minute after VAR Andre Marriner advised referee Anthony Taylor to review the challenge on Saints midfielder Carlos Alcaraz on the pitchside monitor.
Ten Hag was angered by that decision and calls for penalties – a potential foul from Kyle Walker-Peters and handball claim against Armel Bella-Kotchap – as United dug deep to stave off impressive Southampton’s threat in a hard-fought 0-0 draw.
“The inconsistency [means] players don’t know any more what is the policy,” the United boss said on Casemiro’s sending off.
“I think it’s all across. I mean, you see on Saturday, 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 they had “some questions” about the officiating on Sunday, when Casemiro picked up his second red card as a United player.
The 31-year-old was shown the first straight red of his time in Europe’s top leagues in last month’s 2-1 win against Crystal Palace and is now facing a four-match ban.
“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.”
Casemiro is understandably gutted and Ten Hag said “we have to consider that overnight” how to play without him in their next four domestic games.
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 ten men.
“Once again this team showed big spirit, team spirit, good fighting spirit, supporting each other,” said the Dutchman, who downplayed Alejandro Garnacho’s injury after he was seen leaving Old Trafford on crutches with his right foot in a boot.
“We played I think with 11 and with ten a really good level.
“We played in the week, they didn’t. We were with ten and we defended well, we had good counter breaks, we had some good attacks, some good opportunities.
“The strike from Bruno [Fernandes] on the post. Of course I have also seen the strike from them on the post and that was a mistake on counter from a corner from us one-on-one when David [De Gea] saved.
“But all in all the performance was quite well, then you are disappointed you only collect one point.”
Saints shared that frustration after failing to turn a largely dominant performance into a victory as Ruben Selles’ side fight to beat the drop.
“We take the game in two parts,” the Southampton manager said. “The first part is about the result. We are not happy. We talk about coming here and winning the game and do our best to win the game.
“That is what we did so we are not happy with the point.
“But we are really happy with the performance we put on the pitch and we showed the character 11 against 11, and against ten men.
“We showed identity, the principles we have in our game and we saw a team working together, wanting to be together, wanting to get something from the game.
“So, disappointed about the point but very happy with the performance.”