The Scot saw his team take a commanding 2-0 lead at Old Trafford before wilting badly in the second period and drawing 2-2.
Ferguson conceded: “We are lucky to still be in the FA Cup. We ran out of legs. Our two full-backs Rafael [da Silva] and Patrice Evra were just knackered. The two centre midfield players tired badly [Michael Carrick and Tom Cleverley] and we just kept giving the ball away.
“It made it a long day for us because we had to go from one penalty box and back to our own. They were by far the better team in the second half. We could have been maybe four up in the first 25 minutes of the match but I detected there was some signs of tiredness the way we were giving the ball away.”