There was better news about the hamstring injury to Welsh winger George North, who has a "great chance" to play, Robson said, while England centre Manu Tuilagi was progressing well with his shoulder problem.
Although Robson stressed the briefing was a "snapshot", he thought only Irish winger Tommy Bowe, who has a broken bone in his hand, was likely to join Roberts in being ruled out when he reported on player availability to Warren Gatland on Wednesday.
"Jamie's very well this morning, his scans indicate one thing, his demeanour another, (but) he is a 'grave doubt' for this weekend," Robson told reporters at North Sydney Oval.
"Jamie Roberts is not likely to be available, Tommy Bowe is highly unlikely but that was never the plan, other than that everyone else is in line."
Powerful inside centre Roberts, an almost certain starter in the Lions midfield, pulled up with the injury in the 66th minute of the 47-17 victory over the New South Wales Waratahs last Saturday.
Roberts, who qualified as a doctor earlier this year, had expressed doubt that he had suffered a hamstring strain but scans showed one, albeit low on the level of seriousness, which is likely to rule him out of Saturday's match in Brisbane.
Roberts is likely to be replaced in the line-up by compatriot Jonathan Davies, who looks set to start in a midfield partnership with Irishman Brian O'Driscoll.
Bowe, who had always been a more likely prospect for the second test in Melbourne on June 29, was making excellent progress after the removal of his cast, Robson said.
"The small fracture is stabilized, he's in no discomfort and out running in training," he added. "Tommy is confounding us a bit, he already has good grip back and it could be a case of us reining him back a bit."
With a rash of injuries to the Lions backline, Gatland has called up Irish winger Simon Zebo, centres Billy Twelvetrees, Brad Barritt, and their England team mate Christian Wade, also a winger.
All four are in the team for Tuesday's match against the ACT Brumbies, along with former Wales winger Shane Williams, sensationally called up from Japanese club rugby to play just the match in Canberra.
While the long distances most had travelled to join up with the squad were far from ideal, Robson said the players were receiving the best of attention to get them ready for the match.
Coach Gatland said he thought Williams would be fit enough to do the job against the Brumbies and that, even if the target was still to go through the series unbeaten, defeat for the makeshift side would not be the end of the world.
"I've said all along, if we dropped a match or two along the way, I was prepared to sacrifice that to win the test series," he told reporters on Sunday.
"No one is going remember if we win all the other matches and lose the Test series."
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