Schalke have announced they have lodged an appeal with UEFA after Didier Drogba was selected for Galatasaray in Wednesday's 1-1 draw in the Champions League.
The club tweeted: "There are doubts about the validity of the permission to play for Drogba in the Champions League. Schalke 04 reserves its rights and is looking into this."
Drogba, 34, joined the Turkish side in January after claiming his contract with Shanghai Shenhua had been terminated as a result of a failure to pay his wages for three months. The Chinese club issued a statement on their website describing themselves as "deeply shocked" by the move.
Last week, FIFA granted Drogba a temporary licence to play for Galatasaray pending the outcome of its investigation into the validity of his contract dispute.
As reported by Der Westen, Schalke lodged an appeal with UEFA in Nyon within the 24-hour deadline, with general manager Horst Heldt quoted as saying: "It would be reckless not to do it."
Another German newspaper, Die Welt, quotes Schalke finance manager Peter Peters as saying: "Every club has to make sure on their own that their own players have the permission to play."
Schalke have since confirmed via a press release that the matter is with UEFA: "The transfer is said to have been accompanied by problems between the two clubs. There have been reports that claimed the Chinese club had not given the green light for the transfer of the player."
Peters added on the club's official website: "Press releases and media reports led to the assumption that Drogba's permission to play was granted after the end of the deadline for the Champions League squad.
"UEFA separately points out ahead of every game that clubs have the responsibility for all players. UEFA does not review this any further as they rely on the reports from clubs and associations. They only get involved - like in our case - when a club appeals."
There has been precedent for the fielding of ineligible players in the Champions League. In 1992, UEFA ordered a replay between Stuttgart and Leeds United on neutral ground after the German side exceeded the number of foreign players permitted in the second leg of their first-round clash. In 2001, Maccabi Haifa were handed a 3-0 defeat after fielding a suspended player during a 4-0 qualification-round victory over Finnish side Haka, which saw the Israeli club exit the competition.