An error-strewn and indisciplined Carnegie lost by six tries to three as once again they failed to back up an encouraging performance from the week before.
It puts further distance between themselves and the top-four play-off spots and with six games remaining their season is in danger of petering out.
“I’m pretty upset and embarrassed about it,” said the stand-in head coach, whose emphatic conquerors of Nottingham last Sunday had no answer to Rotherham’s game-management, kicking, intensity or discipline.
“Fair play to Rotherham, their intensity was very, very good, particularly over the ball.
“They did to us what we did to them the last time, they hit us with some hard lines, they got over the ball well.
“I’ve got to look at myself and say did I get the selection right after last week? I don’t want to pick on any players because it’s not fair but you wouldn’t have expected that after last week.
“But discipline and penalties are still hurting this team and until we grow up a little bit, we’ll keep losing.”
McGee says he will sit down with director of rugby Chris Gibson to decide whether club punishments need to be meted out to repeat offenders.
“Do we fine players, what do we do?” asked McGee. “I need to address it and I need to address it quickly.
“I need to speak to our director of rugby about how we’re going to go about that.
“There were so many penalties and enforced errors. For me that was the big factor in this game.
“I have to look at the temperament of the group. If it’s calmed and relaxed then you can manage games like this.”
On what the remainder of the season holds for Carnegie, who trail the top four by 10 points, McGee said: “It always has been about building for next season.
“But what we need to look at is trying to keep the faith in what we want to do.
“We showed last week how good we can be.”