The 23-year-old lock was stretchered off during the first half of Scotland's penultimate RBS 6 Nations fixture against Wales on Saturday - Johnson's troops going on to lose 28-18.
Interim head coach Johnson confirmed that Gray had suffered a hamstring injury with the extent of the damage to be made clear following a scan at Spire Murrayfield Hospital.
And, while Scotland doctor James Robson refused to say either way, Johnson is adamant the Sale lock hasn't been seen on a rugby pitch for the last time this season.
"He is in a pretty bad way," said Johnson. "It is more hamstring but we are not sure of the depth of that.
"He will play again this season, I am sure of that. But it will be a significant time away from the game."