The Scottish skipper was forced to pull out of the game against Western Province at the weekend after rolling over on his ankle in training on Friday.
It meant the Lions were forced to ask Ospreys wing Shane Williams to act as emergency scrum half cover for the game. Leicester Tigers' Harry Ellis started and finished the game in the No 9 jersey.
But with Mike Phillips, the leading contender for Saturday's first Test spot still carrying some bruising in his lower back following his man of the match performance against The Sharks in midweek, and Ellis waking up with a slight chest cold the morning after the WP victory, it means the Lions have a small crisis looming at scrum half.
Head Coach Ian McGeechan stopped Blair from training on Sunday and has given the medical team as much time as possible to work on him before Tuesday night's game. If fit he will play.
If he doesn't pass the medical then it is possible Ellis might be asked to back up from his 80 minute appearance on Saturday. The other option is to throw in Williams, who hasn't played at No 9 since his early days at Neath and Amman United, or England centre Riki Flutey, who represented new Zealand Under 19 at scrum half.
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland