Manchester United boss Sir Alex Ferguson admits Antonio Valencia is out for a month with a hamstring injury suffered in victory at Europa League opponents Ajax.

Valencia had only been introduced as a second-half substitute and helped set up Javier Hernandez's match-clinching goal, United's second.

However, he was injured in the build-up and was immediately taken off by manager Ferguson.

"He has a hamstring injury unfortunately," Ferguson said. "It will be four weeks."

Ferguson admitted it was a significant setback given the brilliant form Valencia has been in recently.

"It is a bad blow for us," he said.