Schalke will lodge a complaint with UEFA over the 1-1 draw at Galatasaray in the Champions League because of Didier Drogba's participation.
''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 sports director Horst Heldt and finance chief Peter Peters confirmed they would appeal over the player's eligibility before they boarded Thursday's return flight from Istanbul, the dpa news agency reported.
''It would be negligent not to have it checked,'' Heldt said.
Schalke wants UEFA to examine Drogba's switch from Shanghai Shenhua to the Turkish side - despite FIFA provisionally clearing Drogba to play last week.
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 Shenhua, the sanctions could see Drogba banned and a transfer embargo imposed on Galatasaray.
Schalke hosts Galatasaray in the return leg on March 12.
