Mertens struck the decisive goal against Inter Milan as their 1-1 draw on Saturday saw them reach the final on aggregate.
Koulibaly said, “I've been with him from our first days at Napoli, he used to come pick me up from home as we drove to training at Castel Volturno.
“We had so many laughs in those journeys between Naples and the training ground. His father even spoke to me during my first year there and said he really believed in me, that I should never give up. You can tell Dries grew up in a happy and supportive family.
“I wish him all the best in the world, because he really does deserve it."