Mullins went to ground shortly after half time and had to be replaced by Troy Brown in what looked like a mystery injury as he appeared to black out for a few seconds.
Boss Andy Scott said after the game: "Johnny can't remember anything. We don't know what happened to him but Troy came in and did well.
"At half-time there was no sign that there was a problem, then he went down and just said that he couldn't see anything and can't remember anything, he just felt that the game was going two or three times the speed that it was.
"He was just really disorientated. We had to shuffle the pack very quickly after that and it is something that we will have to look at on the DVD to see if he did get a whack that we didn't notice. It's worrying if not."