The 25-year-old, however, has hinted that he could leave the club, although he is still unsure as to whether he will stay in Serie A.
The Belgian has said that Spain or England would be preferable destinations for his next move and has effectively ruled out a deal with Southampton, despite a big offer from the club.
"I'm still a Cagliari player. Whether I will stay or not depends on the club's vision for the future. If Cagliari have a good plan for my career, of course I will stay," Nainggolan told Goal.com. "The situation is uncertain and everything can happen. But, I have to consider my future career as well.
"It's no problem for me to move to English football for the sake of my career. There are plenty of national team scouts in the Premier League or Spanish Primera Liga, not like in Serie A.
"Honestly, Southampton have made me the biggest offer at the moment compared to the other clubs. But I don't play football for money. My priority is looking for a brighter future for my career and Southampton is my last choice."
Nainggolan joined Cagliari from Piacenza in 2010.