The return of Rio Ferdinand was the crucial factor in taking Manchester United into the Champions League semi-finals, manager Alex Ferguson said on Wednesday night.

England defender Ferdinand missed the 2-2 first-leg draw against Porto through injury and his presence helped United secure a 1-0 win in Portugal, their first clean sheet in six matches.

"We had a challenge, a bit of a battle to come here and win but there were a few differences from last week's match. Ferdinand's return to the side gave us a foundation which has been a great part of our success this season," Ferguson told a news conference.

"In the last few weeks, we've been leaking goals and we had to get back to what got us to this point, to our form earlier in the season, not giving goals away," he added.

Ferguson said the other big difference from the first leg at Old Trafford was that holders United had four days rest before the second leg, two more than last week.

"That really makes a difference, particularly when you come into the tail end of the season," the manager said.

Ferguson singled out the contributions of veteran midfielder Ryan Giggs and young striker Wayne Rooney to the first victory by an English team in a European match in Porto.

"The energy Rooney spends in a game it's refreshing to see a young man with that type of attitude, doing his selfless work for the team," he said.

Ferguson said he was relishing the semi-final against Arsenal.

"It gives it a different type of spice," he said. "It's between two good football teams and it should be a terrific semi-final."