Per Mertesacker will miss most of Werder Bremen's run-in after injuring his knee against Nurnberg.
The German international defender suffered the injury in Sunday's 1-0 win and will undergo surgery on Tuesday.
Mertesacker will be sidelined for between three and four weeks, according to a statement released by the Bundesliga club.
"His loss is a grave one for us," Bremen sporting director Klaus Allofs told reporters.
Bremen are currently two points behind Schalke in the race for the Bundesliga title, with six games remaining.
The Weser Stadion club also have a Uefa Cup quarter-final second leg against AZ Alkmaar later this week.