Nainggolan left AS Roma for Inter last summer.
“I was hoping that I could've done more for the team this season, but the team have done well, and we are close to our objective of qualifying for next season's Champions League," he told Het Nieuwsblad.
“I always feel young and strong, but the injury I had at the start of the season has affected me throughout the campaign.
“Milan is very different to Rome. I can walk quietly down the street here, but in Rome it was impossible."