Sir Alex Ferguson said Manchester United required a wake-up call to overcome Ajax on their Europa League debut but the victory was tainted by an injury to Antonio Valencia that will sideline the in-form winger for four weeks.

Valencia damaged his hamstring 10 minutes after replacing Ashley Young, scorer of United's first goal, as he instigated the move that produced the cushion of a second for Javier Hernández. The Ecuador international was immediately substituted and is expected to miss next week's return against Frank de Boer's side, the league games against Norwich City, Tottenham Hotspur and West Bromwich Albion, and, should United advance as expected in the Europa League, both legs against either Lokomotiv Moscow or Athletic Bilbao. Ferguson confirmed: "He's got a hamstring injury which is unfortunate. Hamstrings are hamstrings, so it could be four weeks."

Two away goals plus a clean sheet in Europe did not completely satisfy the United manager, who admitted the team's first-half display against an Ajax side with only one win this year had left him deeply unimpressed. "I think the result was better than the performance," Ferguson said. "The first half was disappointing. It was difficult to get the rhythm and speed to the game. Ajax made it very difficult for us with their system. They pressed the ball very well. In the second half we had to wake up. We made it difficult for ourselves.

"I just think we didn't speed up our game enough. I know they pressed the ball very well and left their centre-backs to make most of the progress with the ball, but we needed to play quicker. The second half we improved, made some good opportunities and deserved to win. It's a good result. You can't complain too much when you win 2-0 away from home."

The United manager said the finest aspect of the performance was Tom Cleverley's first start in four months plus the return from injury of Phil Jones, Nani and Young. He added: "Winning is always a good habit. We're in a good spell now despite a disappointing first half, but away from home we made a few chances. Bit by bit we have a few players coming back, even though we have lost Antonio Valencia tonight, which is a big blow for us. [Chris] Smalling was on the bench and he will play next Thursday for the return leg. We are getting stronger. Jones has played and Cleverley."

"For Thomas Cleverley there was always going to be a point when we took him off. It was his first game in four months, Ashley Young a second game in two months. Nani and Phil Jones have come back and played a full game which is good."

Hernández had a difficult night but has now scored three goals in his past three appearances for United as he seeks to recapture the prolific form of his debut season. "It didn't go well for him in the sense that the final passes were through in front of him or behind his feet but he was always a threat," Ferguson said. "He proved that again. Maybe he could have also have had a penalty kick in second half. It was difficult to see from where it was. He gave us another option."

Ferguson also praised David de Gea following an encouraging display from the Spanish goalkeeper. He added: "He was confident, assured and controlled his penalty box very well."