Fergie hailed de Gea's superb late save

United led 2-0 after 11 minutes of Sunday's quarter-final but Chelsea hit back to level the score in the second half and the hosts were left hanging on in the dying stages.

Chelsea could have snatched a winning goal in the last minute of normal time, when Juan Mata cushioned a high ball and stepped inside Jonny Evans before blasting a shot at goal.

However, De Gea intervened, spreading himself to divert the ball wide of his right-hand post and thereby keeping United in a competition they last won in 2004.

"It was a very good save," said Ferguson.

"I saw the replay and it was terrific at a very important time, because there was no doubt we would be out if they had scored at that time."

United had appeared to be cruising into the semi-finals after early goals from Javier Hernandez and the recalled Wayne Rooney put them in control on a wintry afternoon at Old Trafford.

Their poise disappeared in the second half though, as goals from Eden Hazard and Ramires brought Chelsea back into the tie, and Ferguson said Tuesday's sapping Champions League loss to Real Madrid was to blame.

"I can't be critical of the side really, as the running they had to do on Tuesday against a top side in Real Madrid had a bearing on this result, there is no doubt about that," he told in-house television channel MUTV.

Chelsea's storming second-half comeback was vindication for interim manager Rafael Benitez, who has endured a torrid time at the club since replacing the sacked Roberto Di Matteo in November.

He had the fans on his back once again on Sunday, with the away supporters audibly questioning his decision to introduce Hazard and John Mikel Obi in place of Frank Lampard and Victor Moses early in the second half.

However, the changes immediately tipped the balance of the game in Chelsea's favour.

Last season's European champions had to endure a long trip to Steaua Bucharest in the Europa League on Thursday, losing 1-0, and Benitez said it was simply a case of managing his squad.

"I'm a professional," he said. "We were going to Romania and I knew we had players who were a little bit tired and we had to manage them.

"We had to think of a starting XI with fresh legs but we knew we'd have to make substitutions, and we did."

United had two extra days to prepare for the game and Benitez said that statistic rendered his side's performance even more impressive.

"They had everything in their favour: playing at home, playing on Tuesday; we were coming from Romania and then travelling again (to Manchester), so we played a great game," he said.

The semi-final draw paired the winners of the tie with Manchester City, but Benitez said it was too soon to think about facing Roberto Mancini's side.

"We have to think about the first game (against United)," he said.

"We cannot think about Manchester City. It will be a tough game."

With United now out of the Champions League, Ferguson said he was not concerned by fixture congestion ahead of the replay.

"We have plenty of time now. We have no European games," he said.

"We have the international break and I don't know when the game will be played. But the only thing we could say is that we are still in the cup."

United winger Nani suffered a miserable end to a forgettable week, limping off before half-time only five days after his match-changing red card against Madrid.

Ferguson, though, played down the extent of the injury.

"That was a blow to us as he was playing very well," Ferguson said.