Sir Alex Ferguson criticised Manchester United for their carelessness during Sunday's dramatic 3-2 victory over Manchester City in the FA Cup.
United were 3-0 up at half-time before City fought back in the second half.
"I think we made them look better than they were," he said. "We were just so careless in the second half.
"At 3-0 up the players thought we were through and took their foot off the pedal. It was a careless performance, we should have been home and dry."
He added: "We set up to hit them on the break and that's what we were able to do with the three goals."
Ferguson could not believe what he was seeing as City pulled two goals back and then had a handball claim against Phil Jones turned down by referee Chris Foy.
"It was ridiculous," said Ferguson. "They went defensive in the second half and waited for us to make mistakes - and we made them.
"Fortunately we scrambled away with a victory from a position where we should have battered them."
Ferguson, who had seen his side lose their two previous Premier League matches, felt Foy was correct to dismiss Manchester City captain Vincent Kompany for a sliding challenge on Nani.
"I think so," he said. "I've seen him do it before, he maybe got off in the past.
"I think if he catches Nani then he's got a problem."
United, record 11-time FA Cup winners, have not won the trophy since 2004 when they beat Millwall 3-0 in the final.