According to an end-of-season profile, Mantalos "endured a difficult start to the campaign", which was interrupted by the need for surgery on a nose broken in training, "but seemed rejuvenated once Manolo Jiménez took over the team" in January 2017. He finished the season with a goal and an assist in a 2–1 win against Panionios in the play-offs that confirmed AEK's place in the UEFA Champions League third qualifying round, and was voted best Greek footballer in the Superleague at the PSAP awards. In light of interest from Spanish clubs, AEK were reported to be unwilling to sell at less than €5 or €6 million, as the player still had two years left on his contract. AEK went top of the Superleague three weeks into the 2017–18 season after a 4–0 win over Larissa, in which Mantalos opened the scoring with a fourth-minute bicycle kick. Although AEK failed to progress in the Champions League, they qualified for the Europa League group stage; in their first match, Mantalos scored as AEK beat Rijeka 2–1. A few days later, he scored an 89th-minute winner as they came back from 2–0 down after 63 minutes to beat reigning champions Olympiacos 3–2. At the end of October, Mantalos suffered another ruptured ACL, this time in his left knee. As a gesture of support, the club offered Mantalos a contract extension until 2022, which he accepted. He returned to action as a substitute on 21 April as AEK beat Levadiakos to clinch their first Superleague title since 1994. Mantalos converted a penalty in a 1–1 draw in the Champions League play-off round second leg against Vidi that, despite having two men sent off, was enough to progress to the group stage on aggregate. Two days after reports emerged that the club had rejected a bid valued at €10 million for Mantalos from English Championship club West Bromwich Albion, he scored his first goal of the 2018–19 Superleague season, a penalty, that gave AEK a 1–0 win away to Asteras Tripolis. His fourth league goal, which opened the scoring in a 2–0 win away to Panionios in January 2019, was controversial as he appeared to handle the ball when controlling the final pass. Mantalos scored twice against Atromitos in the quarter-final of that season's Greek Cup as AEK went on to reach and lose to PAOK in the final for the third year running.