Reigning Turkish Super League champions suffered a humiliating 3-2 defeat against Osmanlıspor to fall 13 points behind league leaders Fenerbahçe.
Umar gave Osmanlıspor an early lead but two goals from Burak Yılmaz and Wesley Sneijder put Galatasaray back ahead.
It was Umar who went onto score the equaliser for the home side just a minute later and former Galatasaray player Musa Çağıran scored the winner on 54 minutes.
Osmanlıspor went into the game with the worst home record in the league having picked up just four points in Ankara all season.
Osmanlıspor 3 – 2 Galatasaray
Referee: Halis Özkahya, Mehmet Cem Hanoğlu, Hakan Yemişken
Osmanlıspor: Ahmet Şahin, Muhammed Bayır, Numan Çürüksu, Uğur Demirok, Pinto, Torje (min. 82 Erdal Kılıçaslan), Musa Çağıran, Lawal (min. 78 Mehmet Güven), Umar, Ndiaye, Rusescu (min. 84 Serdar Deliktaş)
Galatasaray: Muslera, Sabri Sarıoğlu, Denayer (min. 57 Bilal Kısa), Semih Kaya (min. 15 Carole), Hakan Balta, Donk, Selçuk İnan, Sinan Gümüş (min. 71 Umut Bulut), Sneijder, Olcan Adın, Burak Yılmaz
Goals: min. 20 & 44 Umar, min. 51 Musa Çağıran (Osmanlıspor), min. 31 Burak Yılmaz, min. 43 Sneijder (Galatasaray)
Yellow Card: min. 44 Muslera, min. 78 Hakan Balta (Galatasaray), min. 54 Musa Çağıran, min. 75 Rusescu, 90+1 Umar (Osmanlıspor)