The 20-year-old stopper is currently on loan at the Gulls from Portsmouth and boss Johnson is hoping it's not a long lay-off.
"Alex's knee has certainly swelled up a bit, and it looks like some sort of medial ligament damage," he told Devon Live.
"He finished the game and managed to carry on kicking, so hopefully it's not too bad, but we need to find out what's gone on in there."
Bass jarred his knee during the draw with Woking, watch highlights here.
He had a scan yesterday, leaving Johnson with a nail-biting wait for news.
The youngster's season has been blighted with injury - he missed the first four games of the season with a groin injury.
He wouldn't have been eligible for this weekend's FA Cup game either way as his parent club haven't allowed him to play in cup competitions.