Mertens arrived earlier this month from PSV Eindhoven.
"The lifestyle and the food are fantastic," he told Voetbal International. "The language makes everything more difficult with my teammates because only two of them can speak English.
"But I will soon learn Italian.
"We have a team with good players and the company has the desire to do well.
"It's going to be difficult to do better than last year, but Benitez is a great Coach."