Reigning Super Lig champions lost their opening game of the 2019-20 season against newly-promoted Denizlispor at the Denizli Ataturk Stadium.
The Lions suffered a shock defeat in the first surprise of the season.
Fatih Terim gave Ryan Babel, Emre Mor, Seri and Jimmy Durmaz their Galatasaray league debuts but Steven Nzonzi will have to wait to next week to play his first game.
Galatasaray were reduced to 10 men after Marcao was sent off on 43 minutes after receiving his second booking.
The first-half ended in a goalless draw but Denizlispor grew in confidence after play resumed.
Recep Niyaz opened the scoring on 75 minutes and Hugo Rodallega wrapped up all three points with a last-gasp goal in the fourth minute of additional time.
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Galatasaray will take on Konyaspor next in their first home game of the season on Sunday 25 August.
The Lions will be without Marcao who is suspended following his sending off against Denizlispor.
Denizlispor meanwhile take on Antalyaspor on Sunday 25 August in their first away game this term.
Denizlispor 2-0 Galatasaray
Stadium: Denizli Atatürk
Referee: Abdulkadir Bitigen, Süleyman Özay, Mustafa Savranlar
Yukatel Denizlispor: Stackhowiak, Tiago Lopes, Oğuz Yılmaz, Sapunaru, Bergdich, Aissati, Zeki Yavru (min. 82 Mehmet Akyüz), Murawski, Barrow (min. 85 Mbamba), Recep Niyaz (min. 90+3 Mustafa Yumlu), Rodallega
Galatasaray: Muslera, Mariano, Luyindama, Marcao, Nagatomo, Jimmy Durmaz (min. 64 Emre Mor), Seri (min. 46 Donk), Selçuk İnan, Babel, Belhanda, Diagne (min. 72 Ahmet Çalık)
Goal: min. 75 Recep Niyaz, min. 90+4 Rodallega (Yukatel Denizlispor)
Red Card: min. 43 Marcao (Galatasaray)
Yellow Cards: min. 26 Oğuz Yılmaz, min. 28 Tiago Lopes, min. 45+5 Zeki Yavru, min. 90+2 Sapunaru (Yukatel Denizlispor), min. 28 Babel, Dk. 69 Mariano, min. 77 Belhanda (Galatasaray)
