Bayern star Robben a worry for DFB-Pokal Klassiker
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."
Jupp #Heynckes: "The key to success was the team's mindset." #FCBPSG #MiaSanMia pic.twitter.com/PRP9UokiHy
— FC Bayern English (@FCBayernEN) December 6, 2017
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."