Steven Fletcher netted his ninth goal of the campaign in Saturday's surprise 1-1 draw at Tottenham and his form has led to Doyle being relegated to the substitutes' bench as McCarthy perseveres with a 4-5-1 formation.
But McCarthy retains faith in the Republic of Ireland international who he signed from Reading two and a half years ago.
He said: "I've spoken to Doyley and he is cool with it. He is quite low maintenance and he gets on with it.
"He does his work, and continues to do his work, and throughout a career, wherever you are, it just can't be that you continue to be that talisman all the time.
"It needed someone to take a bit of pressure off him because he was the one last season who was playing up front on his own and battling away and scrapping.
"He was playing his international football as well, and he had a knee injury early on in the season.
"All those things contribute to it just not going right for him but what we do know about Doyley is that he was our star man for a while, he is a talented guy and he will come back.
"That's what good players do but we all have tough times. He and Sylvan Ebanks-Blake will start against Birmingham."
McCarthy insists he will take Wednesday night's game seriously despite injuries to several players forcing him to field a much changed line-up from the one which drew at White Hart Lane.
But he is hoping the two teams can produce a much better spectacle than the woeful goalless draw in the first encounter at St Andrew's 10 days ago.
McCarthy said on Tuesday: "The cup does matter and we want to get through. There will be a much changed team tomorrow but there will be a strong team.
"I've already told you Doyle and Ebanks-Blake will play up front. A lot of teams would be thrilled to have those two up front.
"People say the first game was dreadful. It wasn't as good as that. Why the hell is the replay on TV after the first game?
"It was awful but there were mitigating circumstances. Chris Hughton has got Birmingham playing 4-5-1 and being hard to beat, we were playing 4-4-2 and there were no chances, the pitch wasn't great.
"The two teams were bang average. I guess everyone is hoping for a better game. I guess lightning can't strike twice. Let's hope not."
Wolves skipper Roger Johnson has been ruled out of the clash against his former club with an ankle injury.
Full-back Kevin Foley (ankle) and winger Matt Jarvis (hamstring) are also sidelined while midfielder Dave Edwards is rested.