Unal joined NAC on-loan in January and his the ground running scoring 9 times in 12 games for the second-tier side.
The Turkish international striker is likely to stay after City reached a partnership agreement with the Dutch side according to the Manchester Evening News.
City announced a five-year partnership with NAC in April which means the Dutch side will get four and six players from the Citizens between the ages of 18 and 21 each season.
Unal is likely to be joined by City youngster Divine Naah at NAC next season and an additional two players will also join the duo.
City will pay the club a development fee and and pay the wages of the players who join.
NAC missed out on promotion to the top-tier after a defeat against Willem II in the Eredivisie promotion play-offs.
Unal joined City on a a £2,000,000 move last summer but was unable to play in England due to lacking the necessary work permit.