Last week, Kayserispor agreed terms with Spanish giants Atletico Madrid to sign the Ghana midfielder on permanent basis.
Mensah spent the 2018/19 season on loan with the Anatolian Star and impressed.
This prompted the club to agree a three-year deal with the 24-year-old.
Mensah who missed out in the Black Stars provisional squad for the Africa Cup of Nations scored three goals in 21 appearances for Kayserispor last season.