Schalke has lodged a complaint with UEFA over the 1-1 draw at Galatasaray in the Champions League because of Didier Drogba's participation.
Drogba played in the last-16 match on Wednesday, a week after FIFA provisionally cleared his transfer from Shanghai Shuenhua to the Turkish side.
"There are doubts over the valid eligibility for Drogba in the Champions League. Schalke reserves its rights and is looking into this,'' the club said in a tweet on Thursday.
Schalke is asking UEFA to examine Drogba's transfer.
"It would be negligent not to have it checked,'' said Schalke sports director Horst Heldt in comments reported by the dpa news agency as he boarded the return flight from Istanbul.
Shanghai Shenhua complained to FIFA that Drogba broke his contract. Drogba, who had a contract through 2014, claims he was not being paid his salary.
If FIFA rules for Shanghai, the sanctions could see Drogba banned and a transfer embargo imposed on Galatasaray.
UEFA confirmed it received Schalke's protest and that its control and disciplinary body "will consider it.''
Schalke hosts Galatasaray in the return leg on March 12.