Overlooked for the stand-in captain's role, Courtois refused to play in June's European Championship qualifier against Estonia, according to coach Domenico Tedesco who this week said he had since had no contact with the Real Madrid goalkeeper.
Courtois tore knee ligaments and underwent surgery in August, keeping the Real Madrid keeper on the sidelines for six or seven months and delaying any resolution, but earlier this week Belgium full-back Timothy Castagne said he did not think Courtois could return to the squad without any explanation.
Lukaku, however, expressed a different opinion on Wednesday.
"I mean from the bottom of my heart: we must put what happened behind us," he told a press conference as Belgium prepared for their next qualifier against Austria in Vienna on Friday.
"The day Thibaut decides to return, he will simply return. If that happens, he will still be of great value. Thibaut will then answer for himself in the dressing room. You know, I would like to call on the media not to constantly write that Thibaut Courtois or any other player said this or did that.
"We need to put those matters to rest. I wish Thibaut a good rehabilitation, that is the most important thing now, and if he returns, I will be the first to welcome him with open arms."
Lukaku was chosen ahead of Courtois as captain in June, standing in for the injured Kevin De Bruyne.
"That captain's armband is symbolic. There are several players here who can take that kind of responsibility on their own," Lukaku added.
In a newspaper interview earlier this week, Castagne questioned whether the spirit of the squad should be sacrificed for one player, even if he was one of the world's top keepers.
"It would be strange if Thibaut came back as if nothing had happened. It is a problem to be resolved between him and the coach. If he no longer wants to come, it will be his choice," he told the French language daily Le Soir.
But Castagne has since apologised for the comments and spoken to Courtois.
"I did not have my hand slapped for this interview, but my timing was not right," said the Fulham defender. "I've also talked about it with Thibaut. What was said there? That remains between us."
Information from Reuters was used in this story.