The 22-year-old, who has four caps for Germany, sustained the injury during last week's 3-2 defeat by Wolfsburg.
Fifth-placed Leverkusen face local rivals Borussia Moenchengladbach on Saturday. Gladbach are third with 48 points, having taken only one point and failed to score in their last two outings.
Austrian teenager David Alaba has the potential to become an outstanding midfielder although he is currently playing at left-back for Bayern Munich, according to coach Jupp Heynckes.
The 19-year-old has become a regular for his club and at international level this season and Heynckes said he had had to tell the Austrian to take it easy.
"David is burning, he has so much ambition," Heynckes told Bayern's website, adding that he saw a similarity between Alaba and former West Germany player Guenter Netzer.
"One needs a lot of experience to play consistently well in midfield at the highest level," added Heynckes.
Leaders Borussia Dortmund's impressive run is little consolation for Croatia forward Ivan Perisic, who has found himself repeatedly on the bench.
"I came here to play, not to sit on the bench," Perisic, in the queue behind Mario Goetze, Kevin Grosskreutz and Jakob Blaszczykowski for a regular place, told Kicker.
"When I see how we win as I sit on the bench, it kills me. I do not want to wait until someone is injured to play. I just want to play. And if not at Borussia, then at another club."
Hamburg cannot possibly be relegated as long as Thorsten Fink is in charge, according to former Bayern Munich and Borussia Dortmund coach Ottmar Hitzfeld.
"I guarantee it, as long as Fink is coach of Hamburg, they will not be relegated," Hitzfeld, who has won a total of seven Bundesliga titles in stints with Borussia Dortmund and Bayern Munich, told Bild newspaper.
"In spite of the many injuries and suspensions, I believe they will win," he added ahead of
Hamburg have slipped to 14th in the 18-team table after a run of poor results under former FC Basel coach Fink. They have 27 points while Freiburg are 17th, in the relegation zone, with 22.