Only central defender and acting skipper Christophe Berra was retained from the side which drew at Tottenham and a crowd of barely 10,000 greeted the 1-0 replay reversal with boos at the final whistle.
But McCarthy felt the side he fielded was still good enough to progress in the competition and admits his replacement players did not do themselves justice.
He said: "I know the questions will be asked about the team and, if people want to point the finger at me, then so be it.
"But I thought that team against Birmingham should be good enough to win at home.
"Every single one of them were Premier League players.
"I had four players that were definitely injured. I was also never going to play Steven Fletcher or Wayne Hennessey, and I made the other decisions.
"If I picked the wrong team, then maybe one of the players who came in should be telling me, because they all want to play and are ready to play.
"They are all good enough and good footballers in the Premier League."
McCarthy added: "Just because players aren't involved sometimes, it doesn't mean that I forget them and I'm not that shallow that I only consider the eleven.
"It is an opportunity when they get a chance to play but we weren't as good as we should have been, that's for sure.
"I know they can all do it but individually and maybe collectively they maybe didn't do as well as they could have done.
"But they've to be in my thinking. We've got 17 games left and there is no point discounting anybody because they haven't played as well as they can."
McCarthy is optimistic of having a trio of players available after injury for Saturday's home derby with Aston Villa in skipper Roger Johnson, right-back Kevin Foley and winger Matt Jarvis.
But midfielder Dave Edwards, who recently returned to action after a groin problem, is rated doubtful.
McCarthy said: "I expect a positive response to this and there will be changes on Saturday.
"It will be a different team that plays against Villa.
"If I haven't got everyone fit for the game then, of course, there will be others considered.
"Hopefully we'll have the ones available who were out against Blues but all the players need to support each other because it's a big, important game.
"I think I'll certainly have Roger (Johnson) and Foles (Foley) and Jarvo (Jarvis) but Dave Edwards is probably the bigger doubt of them all. "
Berra said: "We have to move on from this defeat and the bigger picture now - and the more important game - is Villa on Saturday.
"We'll come back having rested our legs and we'll be fresh, so I'm sure we'll get three points.
"If we win on Saturday, then the cup game will be forgotten about.
"We have to take on the performances we've put in against Arsenal, Chelsea and Spurs recently."