The Belgium international joined from PSV Eindhoven for 9.7m.
"I am not afraid of competition," he told Gazet van Antwerpen.
"Benitez was determined to sign me, he knows my qualities and I will be able to make my mark here.
"There are some great players at Napoli and I'll try to give my best, just as I did at PSV. It wouldn't be a problem if I sat on the bench a few times, as I'm sure I won't lose my place in the Belgium side."