The 25-year-old has scored 11 goals in all competitions this season, contributing five assists to help Leverkusen to second in the Bundesliga.
But, after suffering the setback in Saturday's 1-0 victory over Hertha Berlin, Sam will definitely be absent for Wednesday's match against the English champions.
"An MRI scan on Sunday morning confirmed the worst: Sam suffered a torn muscle in his left thigh in the match at Hertha BSC," a statement on the club's official website said.
"The Germany international is ruled out for the next few weeks and he is unavailable for Wednesday's Champions League clash against Manchester United."
Sporting director Rudi Voller admitted the loss of Sam was a big blow for Sami Hyypia's side, especially with the match against United on the horizon.
"Sidney's injury is bad news," he said. "It's the biggest game of all the matches in our Champions League campaign. We need to get stuck in and give 100 per cent despite Sam being ruled out."
The fixture pits first against second in Group A but third-placed Shakhtar Donetsk, who meet Real Sociedad, are poised to pounce on any slip-up from either side.
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