The Ecuadorian picked up the injury in the Old Trafford side's Europa League win over Ajax last night.
Red Devils boss Sir Alex Ferguson confirmed the injury blow in his post match press conference and admitted the in-form winger will be missed.
“He has a hamstring injury unfortunately. It will be four weeks," Ferguson is quoted saying by the club’s official website.
“It's a blow to lose Valencia tonight, but bit by bit, we are getting stronger with Nani and Ashley Young coming back.”
Valencia will most certainly be missed as he has become a key member of the United squad in recent months, arguably being their most impressive performer so far this season.
The 26-year-old was voted the Player of the Month twice in a row by United fans after making crucial contributions to a number of the club’s recent matches – i.e. the win at Arsenal and the home victory over Stoke City.
Ferguson’s side will be glad of a rest between now and the return leg next Thursday, as it is FA Cup weekend and they were knocked out by Liverpool in the last round.