The pair sparred throughout Real Madrid's 1-1 draw with Manchester City at the Etihad, and one moment saw the Spaniard send his English counterpart flying into an advertising hoarding.
That wasn't all for that incident though, as when Carvajal went to see Grealish, the City winger threw an arm his way, and the fullback flopped onto the turf.
Explaining the fracas, talkSPORT host and Rangers legend McCoist said: "The ball was going out and Grealish was half going for it but had no chance of getting it.
"Carvajal came in and effectively shoved him and could have hurt him because he went into the hoardings behind the goal.
"He went down there and Grealish kind of lifted his hand a little bit and Carvajal went down like he'd been knocked out by Mike Tyson, I think even one of the Real Madrid players came and picked him up because it was embarrassing.
"It was one of the most embarrassing things I've seen on a football pitch for a long time, he should've [got booked] 100 per cent.
"I thought the referee had a good game but I was looking at his reaction and he was like 'come on get up' he should've booked him."
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McCoist added: "I thought Carvajal was completely out of order, he really annoyed me.
"I thought it was stupidity from Carvajal, I really did, and then I thought his actions can only be described as embarrassing."
Grealish was asked about his opponent on the left wing post match, and admitted he'd been warned before the tie.
He said: "Playing against Carvajal, I can't count the number of people who said to me before the game 'make sure you don't… you know what he's going to be like'.
"It was a good battle, I really enjoyed it out there. But I was tired at the end, I've never had cramp before but I had it in both my calves at the end."
Carvajal, meanwhile, revealed he was slightly frustrated with his team's approach.
"It wasn't the plan but we did respect the rival and we were trying not to concede at the start and it took its toll on us. We woke up a bit later than usual."