The former Chelsea and Toffees forward posted a selfie while on holiday in sunny California.
Nainggolan then commented “Tot snel” with a laughing emoji, which in Dutch translates to “see you soon.”
Antonio Conte was recently appointed as Inter’s new boss as he looks to build on the foundations laid by Luciano Spalletti, which will see them play in the Champions League for the second successive season.
The Italian coach wanted Lukaku while in charge of Chelsea, but instead got Alvaro Morata from Real Madrid after United won the race for the 26-year-old.
Man Utd are holding out for £75m for Romelu Lukaku as Inter prepared an offer (Image: GETTY)
Radja Nainggolan dropped a transfer hint on the Man Utd striker's instagram (Image: Instagram)
At the start of the month, he hinted that his time under Ole Gunnar Solskjaer at United was done.
"It is good that Conte went to Inter, for me he is the best coach in the world,” he said.
"As for my future, I have already made a decision that I still cannot communicate publicly out of respect for Manchester United, with whom I have a contract."
Lukaku has already previously flirted with a move to Italy on more than one occasion, a country that has a history with iconic strikers.
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