Royal Antwerp midfielder Radja Nainggolan says Romelu Lukaku should never have returned to Chelsea.
The pair played together with Belgium.
Nainggolan said: "I told Romelu that he has made a bad decision in leaving Inter. There he was the king, and perhaps the best striker in the world.
"But he was unable to say no to the club he loved. This is the third time that he has gone to Chelsea - and the third time that it hasn't worked for him.
"He says that the team was built around him, but that the system hasn't worked. So the coach [Thomas Tuchel] has opted for other, smaller players who run around a lot.
“And the results have come as a consequence. Had he stayed at Inter, with a team built around him, he would have remained among the world's best strikers for another four or five years."