At the same time that Everton appointed Frank Lampard as their new manager, the Toffees also pushed through the transfer of Alli from Tottenham Hotspur.
The hope was that the England international would rediscover the form which had made him one of the most highly-regarded creative midfielders in Europe earlier in his career.
However, since his arrival at Goodison Park in January, the 26-year-old has been provided with just one start and 12 substitute appearances in the Premier League.
In total, Alli has featured for just 367 minutes in an Everton shirt, and there have been further signs since the start of the new campaign that he will not break into Lampard's starting lineup.
As a result, Everton made the decision to accept a loan approach for Besiktas, ultimately leaving the final decision up to the player.
Alli has now opted to take the opportunity to try to revive his career in Turkey with a team who are bidding to get their hands on the league title.
Besiktas have collected seven points from their opening three fixtures in the Super Lig, leaving them in third position in the table.
Alli will link up two players that he knows from his days with Spurs with Gedson Fernandes and Georges-Kevin N'Koudou both at Vodafone Park.
According to transfer expert Fabrizio Romano, Besiktas possess two different options when it comes to signing Alli on a permanent basis.
If triggered before the start of 2023, Alli can leave Everton for £6m, with that rising to £8m if it is triggered between January and June.
With Besiktas not back in action until Monday, Alli may feature in the squad for the league meeting with Sivasspor.