The Serbian made an instant impression upon replacing Papiss Cisse in the 75th minute, launching a scything challenge on Matt Targett which earned him a yellow card from referee Craig Pawson.
Mitrovic's actions drew criticism from Saints assistant Erwin Koeman - standing in for injured brother Ronald - who insisted: "I don't think he (Mitrovic) touched him, but if he did then Matthew was in hospital, I think."
Steve McClaren: "We were quite fortunate he did not get a red there and there would have been no complaints if he did.
"I don't know if he got that much contact but I'm just saying with the present rules of the game... I think in the olden days those kinds of challenges were 10-a-penny.
"That's what he (Mitrovic) is. He certainly doesn't hold back. He wanted to come on and impress. He's only a young lad and you can see how strong and direct he is. He is going to be a player."