You could say that Casemiro was lucky not to receive a second yellow card.
After being warned moments earlier, he fouled Luton's Ross Barkley.
But not every foul is a yellow card and perhaps that was his last chance.
Erik ten Hag brought him on during half-time before he was sent off for his next one A violation would have resulted in a red card.
You have to see four or five careless fouls before you get a warning for persistently breaking the rules.
Ten Hag revealed in his post-match interview that he replaced Casemiro due to what he believes is unfair treatment by referees.
He said: “Even if he doesn't touch opponents, he gets a yellow card.
“Often the first moment of the match.
“I think it's crazy and it was so unfair that first booking.
“That second could have been like that, that's why I took it off.
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“He hits an opponent and he gets a yellow card.”
Former Real Madrid player Casemiro has seemingly turned his poor form around in recent weeks with a string of impressive performances.
Ten Hag said of the Brazilian: “You can see that he is now back in the team and that he gives so much peace and organization.
“I'm really happy that the team has taken it to the next level since he's been back.”
Casemiro and his Manchester United teammates will be back in action and welcome on Saturday afternoon Fulham to Old Trafford.