Fellaini, 29, joined United from Everton in the summer of 2013, and the midfielder has scored 15 times in 133 appearances for the Red Devils in all competitions.
The Belgium international made 28 Premier League appearances last season after working his way into the plans of head coach Jose Mourinho, but according to Ergun, Fellaini will depart ahead of the 2017-18 campaign.
"Fellaini will leave Manchester United and he is now in our radar," Ergun told Ajansspor. "There will be surprises, we are in talks with Fellaini.
"We have made an approach, we are close and if we get Fellaini we will end our pursuit of Alfred N'Diaye. We need a player like Fellaini who can move around in midfield."
United will continue their preparations for the new season with a clash against Norwegian side Valerenga in Oslo on Sunday night.