The Belgian midfielder only joined Inter in June from Roma in a deal reported to be worth €38 million (£34m/$43) plus Nicolo Zaniolo, though he has failed to live up to expectations.
Nainggolan has scored two goals in 11 Serie A outings, with Inter currently sitting third in the table.
But he looks unlikely to feature in their next few games following Inter’s shock announcement that he will be barred from “football activity”.
Inter’s statement read: “Inter can confirm that Radja Nainggolan has been temporarily suspended from football activity for disciplinary reasons.”
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— Inter (@Inter) December 23, 2018