"I think whoever plays in goal against Chelsea could be the one who starts in the final - but I don't think a decision has been made yet," he said.
"It's a bit difficult really, but when Roy Carroll came in and United got to the FA Cup final, Sir Alex did say he was going to give Tim Howard another couple of games before then.
"He has a very difficult choice. It is one of those difficult situations.
"There is only one position when you are a goalkeeper and we have two very good keepers at the club."
Carroll played throughout a six-game spell which saw United close the gap on second-placed Chelsea and beat Arsenal in the FA Cup semi-finals - but Stepney feels Howard has also done enough to warrant selection for the big game against Millwall on 22 May.
"I just feel Sir Alex left Tim out because he thought he was tired," he added.
"He has played more games than he has ever done in his life because in America they don't play that amount of games.
"We saw that he was tired, but he has come back refreshed and in the two games he has played he has only conceded one goal and that was a penalty.
"Like any goalkeeper, and that includes Roy Carroll, he wants to play and he has to prove to Sir Alex that he is the one to pick."