Bayern star Robben a worry for DFB-Pokal Klassiker

Arjen Robben could miss Bayern Munich's DFB-Pokal clash with rivals Borussia Dortmund after head coach Jupp Heynckes was unable to nail down a return day for the winger.

Robben last featured in the 2-1 Champions League win at Anderlecht, where he was substituted due to what was described at the time as a "minor calf strain".

He sat out the Bundesliga matches at Borussia Monchengladbach and against Hannover, along with Tuesday's concluding Group B win against Paris Saint-Germain and Heynckes does not expect the 33-year-old to return for Saturday's trip to Eintracht Frankfurt.

"Arjen has problems with the sciatic nerve which is a bit inflamed," he told reporters.

"Normally it's not a severe injury but a thing of one or two weeks, but we are not able to make a forecast."

Bayern play Stuttgart in their last Bundesliga fixture of the year before heading into the mid-season break by entertaining Pokal holders Dortmund on Wednesday, December 20.

The Bundesliga champions won the league encounter between the sides at Signal Iduna Park last month 3-1.

That victory maintained the feel-good factor enjoyed at Bayern since Heynckes returned to the club to succeed Carlo Ancelotti until the end of this season.

The 2012-13 treble winner confirmed last week he was still working under a gentleman's agreement having not signed a formal contract – a state of affairs he remains relaxed about.

"About the contract, for me a handshake means the same - especially with Bayern," he added.

"That is for definite too. When will I sign my contract? Maybe it will be a Christmas surprise or something like that. But for me that's not so important."