Spain midfielder Thiago Alcantara, who has been linked to Liverpool and Manchester United, took part in Bayern Munich's first full training session on Friday in preparation for the new season.
With a year left on his contract, the 29-year-old has stalled over signing an extension with the Champions League winners having been linked to the two Premier League sides.
Club bosses say the central midfielder has told them he wants a "fresh challenge" after seven years in Munich.
However, last weekend Alcantara stated "I have never said I want to leave".
Holders Bayern kick-off the new Bundesliga season next Friday by hosting Schalke 04 with Alcantara one of several players whose future is unclear.
Austria defender David Alaba, who also trained Friday, is another stalling over extensions talks.
The 28-year-old, the only product of Bayern's academy in the starting side, is reportedly seeking a salary raise to earn around 20 million euros ($24 million) per season.
Likewise, midfielder Javi Martinez, 32, whose contract also expires in 2021, has been linked to a transfer back to Spain or a move to China.
On Monday, Bayern head coach Hansi Flick, who missed Friday's session to attend a funeral, admitted the uncertainty makes planning "not quite so easy".
However, as long as first-choice stars Alaba and Alcantara are in his squad, "I'm planning with them".
Bayern have already lost Ivan Perisic, Philippe Coutinho and Alvaro Odriozola - all signed on loan for a season - who were part of the squad which won the Champions League final on August 23.
Flick knows Bayern must "make up for" the loss, especially as the European champions face a punishing opening schedule.
"We will need good quality across the board," Flick added as Bayern face Sevilla in the UEFA Super Cup on September 24, then Dortmund a week later for the German Super Cup.
The 55-year-old wants to finalise his squad - "I hope October is soon" - with the transfer window closing on October 5.
Sports director Hasan Salihamidzic is looking for a right-back, to replace Odriozola, and another winger to back up Serge Gnabry, Leroy Sane and Kingsley Coman.
Flick has also said Sane, signed in July from Manchester City, must improve his defensive game.
"We will continue working intensively when not in possession. He must also get involved. This will be a change for him," Flick added.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)