On Tuesday, Besiktas confirmed they had signed the Ivorian defender after he was deemed surplus to requirements by Erik ten Hag. United had chosen not to give Bailly a shirt number and left him out of their pre-season tour squad.
MEN Sport understands the Reds allowed Bailly to leave on a free transfer even though he had a year remaining on his contract so as to free up his wages. Taking to X on Wednesday evening, the 29-year-old released a tribute to the fans.
He wrote: “As I leave this club after many years of incredible memories, outstanding teammates, and trophies, I want to express my profound gratitude. This chapter may be closing, but the friendships and memories will forever remain in my heart.
“The fans here are the real champions.”
United spent 30m on Bailly in 2016 as they looked to bolster their defensive options.
Bailly would go on to spend seven years with the Reds and made 113 appearances during this time. However, the centre-back rarely threatened to make a breakthrough as a permanent starter at Old Trafford.
Last season, Bailly joined Marseille on loan with the French giants inserting an option to purchase the defender in the summer. But, after 23 appearances for the club in all competitions, Marseille opted not to activate this clause.