Mertens claimed another winners' medal on 22 December 2014 after Napoli's penalty shoot-out victory over Juventus in the Supercoppa. Despite being predominantly used as a substitute, he continued his fine form for Napoli over the course of the next season and a half, netting 10 goals and contributing 11 assists across all competitions in 2014–15, and 11 more goals in 2015–16. His tally in the latter campaign included a brace in Napoli's joint-largest ever winning margin in the Europa League, a 5–0 win over Belgian side Club Brugge. Mertens also scored a second-half hat-trick in a convincing 6–0 victory over Bologna on 19 April 2016.