The Liverpool legend has never been the Manchester United midfielder's biggest fan, and had the chance to explain why after the 2-1 win.
United blasted past Luton with two goals from Rasmus Hojlund inside seven minutes, but after Carlton Morris pegged them back, it was a case of holding on.
That became complicated when Casemiro avoided an early booking, but then got a yellow in the 34th minute.
Playing the rest of the half on the edge, Erik ten Hag removed him from the action at half-time for Scott McTominay, and Souness wasn't surprised.
"This is not new, this is how he played at Real Madrid," the talkSPORT host said.
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"He was famous for professional fouls, stopping breakaways, committing fouls and taking a card for the cause because of a threatening situation, that was his career at Real Madrid.
"He's a steady player. He doesn't put it about, he gives fouls away, he sees danger and sometimes he's a bit clumsy, he's 31 now, but you wouldn't say he puts it about.
"The biggest problem he had yesterday, which is part of the professional game and not a new thing, the biggest danger of him getting a second yellow was from the Luton players going at the referee, but that's not a new thing either."
The comments may sound familiar to some United fans, with Souness calling Casemiro a 'steady Eddie' upon his £70million arrival in 2022, and commenting that he 'was never a great player'.
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Those comments seemed to have aged badly when Casemiro helped the Red Devils into the top four and saw them win their first trophy in six years with the League Cup.
However, as the season wound down, Souness' argument came back up as Casemiro's impact began to dwindle, and it was the same this season with an injury halting his progress.
Still only 31, the Brazilian signed a four-year contract at Old Trafford, and former West Ham and Newcastle manager Alan Pardew explained that defensive midfield isn't the best position to start showing your age.
The talkSPORT host added: "When you're a central midfield player and the one breaking up attacks and making tackles you actually age quicker than a flair player.
"If the timing is just off and you can't make it, you're making more fouls, Casemiro is in that world in my opinion. I think it was a good decision personally [to take him off], I would have done the same thing.
"He could have got sent off, he was that close, the bottom line is he had to take him off at half time because they were in danger of going down to ten men, you're talking about a very experienced player who shouldn't be in that position."