Darron Gibson's first half brace gave Sir Alex's Ferguson's side a 2-0 win at Old Trafford on Tuesday night in a game that Redknapp thought was played like it was a friendly match.
"I'm very disappointed," Redknapp told Sky Sports.
"We are never going to get a better opportunity than we had in the first-half tonight. I think they had two shots on goal and scored twice. We had a lot of the ball, but never really made it count. I was disappointed.
"We have been playing so well of late, but there was no atmosphere, it was like a practice match.
"There was no atmosphere in the stadium and there was no atmosphere in the game. It was played at a lethargic pace. I don't think there was a tackle made in the first half.
"I've never been to Old Trafford when there has been no atmosphere. It was like it never got going. Nothing was happening.
"It's a cup quarter-final, but there wasn't a tackle first half. It was like a friendly. It was strange. I haven't experienced an atmosphere like that for a long time.
"We had good opportunities, they had two shots at goal and scored twice. They punished us and we haven't punished them when we had the opportunity.
"It became difficult second half, but the whole game had a funny feeling for me. It was not like a league game, when the atmosphere is electric, it was a strange night."