The 25-year-old lasted just four minutes of Bayer's 1-0 win at Hertha Berlin on Matchday 13, with subsequent tests revealing a torn muscle in his left thigh.
'Game of games'
"His absence is very bad news for us," said Leverkusen sporting director Rudi Voller. "This is the game of games for us in the Champions League so we have to give it our all even without Sam."
Leverkusen are currently second in the group, a point behind English Premier League champions United, and two clear of third-place Shakhtar Donetsk.