The striker has endured a frustrating start to his second spell with Inter after returning on a season-long loan from Chelsea.
Lukaku has made only five appearances for the Serie A club this season, spending two months on the sidelines with a thigh injury before suffering another setback last month.
The Red Devils' record goalscorer is not ready for the encounter with Canada at Ahmad Bin Ali Stadium on Wednesday and Martinez says there are no guarantees he will be ready for the Group F matches against Morocco and Croatia.
The Belgium head coach said on Wednesday (AEDT): "We're very happy the way that he is progressing. He had a very good session yesterday, not with a group, but is progressing in a steady way. We haven't got a timeline.
"I wouldn't be able to tell if he's going to be the second game or the third game. If I follow the medical advice, at this point it would be the third game. If I follow how the player feels, probably we’ve got a chance for the second game."
Eden Hazard has also had fitness issues since making a big-money move to Real Madrid, where he has failed to make the impact that was expected of him.
Martinez is backing his captain to show what he can do on the international stage once again.
"It's been a tough situation in terms of the playing minutes," the Spaniard said. "We are a better team when Eden Hazard is in our dressing room, that's for sure."