Diagne leaves the Turk Telekom Stadium seven months after he moved to Istanbul on a four-and-a-half-year deal from Kasimpasa.
He joined Galatasaray in January and he went on to finish as the top scorer in the Turkish Super Lig for the 2018-19 season with 30 goals in 29 matches.
Prior to the temporary transfer, the Senegal forward played in every one of the Lions’ opening league matches this season but was unable to open his goal account.
The former Tianjin Teda attacker will be looking to help Philippe Clement's side continue their unbeaten start to the 2019-20 season when they take on Cercle Brugge in the Brugge derby after the international break.
Meanwhile, Diagne is looking forward to the Uefa Champions League fixture between Galatasaray and the Blue-Black at the Jan Breydel Stadium on September 14.
"Thanks Galatasaray for everything. Thanks to all my colleagues, thanks to all the fans for cheering and affection," he wrote on his farewell message.
"Even though it was a short relationship, it was intense and full of victories and satisfactions. You are my family forever. See you soon in the Champions League."
Grazie Galatasaray per tutto. Grazie a tutti i miei compagni al Mister a tutti i collaboratori. Grazie a tutti i tifosi per il tifo e per l'affetto. Anche se stato un rapporto breve é stato intenso e ricco di vittorie e soddisfazioni.💛❤️siete la mia famiglia per sempre a presto 👍champion league (Türkiye )❤️💛