​Bayern Munich defender David Alaba is a doubt for the DFB Cup semi-final match against Borussia Dortmund after picking up a capsular tear in his left knee.

Alaba was substituted early on during the 2-2 draw with Mainz on Saturday, during which Arjen Robben and Thiago Alcantara's goals were pegged back by strikes from Bojan and Daniel Brosinski, but Bayern manager Carlo Ancelotti says the issue is not a big one.

"It isn't a very big problem," Ancelotti said, as quoted by ​Bayern's official website.

"We'll see on a day-to-day basis and decide whether he can play."

The injury will not help Bayern as they look to return to form, having suffered a Champions League exit against Real Madrid during midweek and drawing in the Bundesliga on Saturday following Alaba's substitution.

Alaba himself remains positive regarding a quick return, posting on Twitter: "Fortunately my knee injury is not a big deal and I hope to be back on the pitch in some days."

​​Should he fail to be fit in time for the cup tie on Wednesday then Spaniard Juan Bernat could earn a recall. Bernat has been behind Alaba in the pecking order since joining the Bundesliga club in 2014 from Valencia.

He has made 14 Bundesliga appearances so far this season, scoring once and adding two assists while Alaba has figured in 28 league matches, grabbing two strikes and four more assists.

Bayern are undefeated in their last four matches against Borussia Dortmund, with their last loss coming in 2016 when Pierre-Emerick Aubameyang scored the winner.