Nainggolan has been linked with Tottenham and Inter Milan.
"I do not understand your comments," he posted on the social network. "As long as I'm in Cagliari I will always give my all for this shirt.
"I've been linked with many teams, but even before I've gone I'm being called a mercenary?
"Where is the logic in that? Do not forget that I have already turned down a lot of important teams.
"I will never accept being called a mercenary."