Six Shakhtar Donetsk players refused to return to the city following Saturday's friendly in France because of concerns over the conflict in Ukraine.
Donetsk is currently controlled by pro-Russian rebels, with the Ukrainian army attempting to regain control.
Alex Teixeira, Fred, Dentinho, Douglas Costa, Facundo Ferreyra and Ismaily did not fly back after the game in Lyon.
Club president Rinat Akhmetov has warned that the players may face sanctions if they do not return.
"Players have contracts that they have to abide by. If they do not come, I think, they will be the first to suffer," Akhmetov told the club's website.
"Hopefully, the mind and heart will prevail over misunderstanding, and the players will not follow temptation and fear. Especially since there is nothing to fear."
Five of the six players are Brazilian, with Ferreyra hailing from Argentina, and have a reported market value of £46m.
Shakhtar are currently not allowed to play home matches at their Donbass Arena, due to the conflict.
The new season is scheduled to begin this weekend and the Ukrainian Football Federation has yet to decide where the club will play.