Nainggolan was on the scoresheet, though Frosinone threatened at 2-1.
He said, "We thought it was a bit too easy, but these games are above all difficult when you relax. They work hard, are really aggressive and have a lot to lose, so they give everything.
“After it went to 2-1, we did well to hold on to the ball and add another goal to finish it," the midfielder told Sky Sport Italia.
“Unfortunately, I've had a lot of injuries and so far at Inter I haven't been able to give what I know that I can. I hope to start finding my form now."