Former Manchester City boss Roberto Mancini lost his first league game as manager of Galatasaray, with Akhisar Belediyespor beating the champions 2-1.
Galatasaray, who drew 2-2 with Juventus in Mancini's opening match, fell 2-0 behind with Oumar Niasse scoring in either half for the home side.
Former Chelsea striker Didier Drogba reduced the deficit, but it proved nothing more than a consolation.
Gala have not won in five league games and have seven points from six matches.
The 48-year-old Italian's first match was a Champions League Group B draw with Juventus on Wednesday.
City sacked Mancini in May, a year after he led them to their first Premier League title.
Mancini won three successive Italian league titles and two Italian Cups with Inter Milan, before winning the FA Cup and Premier League with City.