On Wednesday, Villarreal announced the signing of Turkey international Unal on a five-year deal.
The 20-year-old spent last season on loan at FC Twente in Eredivisie, scoring 19 goals in 33 matches in all competitions.
"He is a strong player of great stature, but also possesses great technical ability with the ball at his feet: dribbling, change of pace, a knack for getting in behind the defence and attacking the goal," Villarreal said in a statement.
"Strong in the air, Unal also has a powerful shot and is dangerous on set plays, making him the complete footballer at just 20 years old."
Unal moved to City from Turkish side Bursaspor in 2015 but he never played a competitive game for the Premier League giants.
He also had loan spells at Genk and NAC Breda, while he has earned five caps for Turkey since debuting in 2015.