The Red Devils squandered a two-goal lead at home against Chelsea with the Blues netting twice in the second half to force a 2-2 draw and set up a quarter-final replay.
Ferguson admitted that his United players had some fortune late in the piece but was full of admiration for them after a very tough loss to Real Madrid in the Champions League on Tuesday.
"We're lucky to still be in the cup," he told 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."