Mousouris started his professional career at Ethnikos Piraeus, where he played for 4 years before moving to AEL.
At that time, the Thessalian group of companies, BIOCARPET which has taken over the wheel at AEL, since the beginning of the 70s, decided to strengthen the team's roster, in order to elevate the ckub and make them permanent in the first division.
Despite his impressive start, however, Zlatko Čajkovski, who had meanwhile replaced Fadrhonc in the technical leadership of the club, saw in Mousouris other gifts and virtues and he relocated him to the right side of the defensive line.
Although Mousouris preferred playing in the offense, being completely happy with the conditions at AEK and realizing that with the return of the then injured Dušan Bajević and the duo he formed alongside Thomas Mavros in attack seemed unshakable, he accepted the change of his position without any complaints.
Čik's choice was fully justified as Mousouris, possessing great speed and stride and having the attacking skills, developed into an irreplaceable modern right full-back, a pioneer for the era who, whenever was at the offense, he did not miss the opportunity to score.