Dortmund, Germany (Sports Network) - Borussia Dortmund will be without a key player for the next four weeks as it was discovered on Monday that defender Mats Hummels is out with knee and ankle problems.

The Germany international had just returned from a brief spell on the sidelines for Saturday's derby match with Schalke, but he lasted just 45 minutes before being replaced and will now miss a key stretch of games for Dortmund.

Hummels has started 23 of 25 Bundesliga matches for Dortmund, which sits in second place in the league table, and he will now face a race to be fit in time to feature in either leg of the club's Champions League quarterfinal.

"This is bitter news for us as a club and for him as a player," Dortmund manager Jurgen Klopp told Sky Deutschland.

"For us as a club, it's the worst news we could have received at this point in time.

"When he suffered the injury it didn't look that bad. Unfortunately it was."

03/11 13:04:19 ET