Younes is expected to join Napoli for €5m in the coming days.
“He's a very strong player, he takes on his man and beats him," Mertens said on Radio Kiss Kiss Napoli.
“I'm not sure if it's 100 per cent yet, but he's a great signing for Napoli."
The Belgian also remarked: “We've definitely entered the most important phase of the season, it will start on Sunday. We can't look too far ahead, we just have to continue like this as a team.
“Is it between ourselves and Juventus? We know Lazio are doing really well, but so are the others. It won't be easy, but we're just thinking about ourselves so lets think about Sunday now and get three points.
“The Europa League? We want to do well in this competition too, which we take very seriously."