Berlin, Sep 10 (IANS): Four days ahead of their Champions League opener against FC Barcelona, Bayern Munich are facing a test.

In the Bundesliga's fourth round of matches, the Bavarians are crossing swords with national league rival RB Leipzig on Saturday evening.

The Bavarians' trip turns into a special challenge as Bayern coach Julian Nagelsmann, defender Dayot Upamecano, and midfielder Marcel Sabitzer return to feature against their former club.

Leipzig's fans have announced they will give their former heroes a painful welcome.

Leipzig CEO Oliver Mintzlaff, however, has demanded from his side's supporters fair treatment to the returning trio, Xinhua reports.

The Bayern coach spoke about his concerns over Leipzig fans not being overwhelmingly happy to see him again.

Accusations are rife that Bayern once again used their overwhelming financial muscle to crush domestic competition unfairly by signing its nearest rivals' top players. It is something that hurt the fortunes of other clubs in the country too.

RB fans accuse Nagelsmann of having betrayed them after the 34-year-old had promised not to take any of Leipzig's footballers with him.

But soon after Nagelsmann signed his Bayern contract, 22-year-old French international Upamecano followed for 42.5 million euros accompanied by Austrian international Sabitzer for a reported 15 million.

Bayern, for many years, have been confronted with accusations of undermining opponents to secure their leading role on the national stage.