The 29-year-old defender joined United from Villarreal in 2016 and was a key figure under former managers Jose Mourinho and Ole Gunnar Solskjaer.
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However, injury problems have plagued him over recent years at Old Trafford.
And last season he was shipped out on loan to Ligue 1 side Marseille after being deemed surplus to requirements by boss Erik Ten Hag.
Ivory Coast international Bailly, who had a year left on his £80,000-a-week deal, was made available for transfer by United this summer as they looked to raise funds.
The ace had been in talks to link up with former team-mate Cristiano Ronaldo at Al-Nassr but that deal broke down.
Bailly held discussions to terminate his contract ahead of a free transfer move but we reported how he had allegedly demanded a loyalty bonus owed to him of £600k – which the Red Devils were unwilling to pay.
Fulham were then linked with a swoop, but now it will be Turkey where he will be looking to resurrect his career after Besiktas announced his arrival.
They tweeted: “Welcome to Besiktas Eric Bailly.
“Our club has reached an agreement with Manchester United for the final transfer of Ivorian defender Eric Bailly.
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“We wish Eric Bailly, who we believe will provide important services to our club, great success with our glorious jersey, and present it to the public with our respect.”
Bailly made 113 appearances for United, scoring once, and won both the Carabao Cup and Europa League during his first season at the club.
Details of his new contract are still yet to be confirmed by the Turkish giants, although it has been widely-reported it is only an initial 12-month package.
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