Schalke 04 have appealed to European football governing body, UEFA, over Galatasaray striker Didier Drogba's participation in Wednesday's 1-1 Champions League draw in Istanbul.
Drogba made his full debut in the 1-1 draw in Istanbul following his January move from China.
"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," the club said on their official Twitter account.
UEFA later said in a statement it had received "a protest concerning Drogba's eligibility" and an investigation will now be held.
Ivorian striker Drogba joined the Turkish club in late January for 18 months from Shanghai Shenhua.
The Chinese club said his transfer had violated the terms of his contract though, and that Drogba was still their player.