The Reds winger, voted United's Player of the Month for January, was hurt while helping to create the second goal at the Amsterdam ArenA, scored by Javier Hernandez.
Valencia was immediately withdrawn from the action and replaced by Danny Welbeck for the final few minutes. Sir Alex Ferguson later confirmed the nature of his injury and the likely length of his absence.
"He has a hamstring injury unfortunately. It will be four weeks," said Sir Alex at his post-match press conference.
"It's a blow to lose Valencia tonight, but bit by bit, we are getting stronger with Nani and Ashley Young coming back.
"Tom Cleverley has played a bit of football tonight and Phil Jones has played a full game which is good for us. Chris Smalling was on the bench but will play in the return match at Old Trafford next Thursday."