The former Roma defender made the move last week.
Mertens said, “Manolas is very good, but we've lost (Raul) Albiol, who was very important. (Jordan) Lukaku is very good.
“He's still young, he wants and can still learn so much, so he'd be a great purchase.
“When the fans love you, it helps and makes your life easier. (Lorenzo) Insigne and I are different. He has a family, two children, is often at home and doesn't go out.
“I go out a bit more, but on the field Lorenzo is very important."