The Reds were deservedly 2-0 up at half-time in Sunday's quarter-final clash and looked all set for a Wembley date in April. But an injury to Nani just before the break was the beginning of a bad conclusion to the match as Rafael Benitez's side came back to level at 2-2 and threatened on several occasions to win the tie at the first time of asking.
"We're lucky to still be in the cup," Sir Alex admitted on MUTV. "We just ran out of legs and Chelsea were far the better team in the second half. But I can't be critical of my side because the running they had to do on Tuesday night, against a really top side in Real Madrid, had its bearing on today's result, no doubt about that.
"The two full-backs, Rafael and Patrice [Evra], are knackered and the two central midfielders tired badly. We kept giving the ball away and it made it a long day for us, going from one penalty box and back to our own."
Few observers would have expected a replay after seeing Javier Hernandez and then Wayne Rooney add to their growing collections of goals against Chelsea, and the Reds might have been even further in front with a bit more fortune.
"We could have been 4-0 up in the first 25 minutes of the match," remarked Sir Alex. "The two strikers' goalscoring has been great this year - that’s Wayne on 15 now for the season and Chicharito on 16. But I think we needed that lead because I could detect towards the end of the first half that the players' legs were starting to go."