Hernandez suffered the injury in the first half of United's 1-0 victory at Aston Villa. He will miss Wednesday's UEFA Champions League game against Basel and the next month of the season. The defending English Premier League champions play seven games in the next four weeks.
"I think it is torn ligaments in the ankle and he'll be out for four weeks," Sir Alex Ferguson said. "We are having a bad time with injuries at the moment but that is why we have a strong squad.
The injury to Hernandez compounds Ferguson's selection headaches. He is already without midfielders Tom Cleverly and Anderson. In recent weeks, he has used forward Wayne Rooney in the midfield. Danny Welbeck is the only available first-team striker that has seen significant playing time in recent months. Ferguson plans to take a patch-work approach to the attack in the coming weeks.
"Danny Welbeck is fit again and fortunately he got 15 minutes [against Villa] which is good, so he is an option for Wednesday," Ferguson said. "He has not had a lot of football recently which is something I have to consider.
"But Nani did alright through the middle and Antonio Valencia did very well for us when he came on, he was excellent. So I have some options up-front. We may have Berbatov fit for Wednesday also, but we are getting our fair share of injuries at the moment."
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