Blair rolled over on his ankle in training on Friday and was pulled out of the game the next day against Western Province as a precaution. He has received intensive treatment ever since.
Leicester Tigers and England scrum half Harry Ellis played in the 26-23 win at Newlands with Shane Williams acting as cover on the replacements bench. Williams started his career as a scrum half at Amman United.
The problem for McGeechan as he approaches Test week is that Welsh No 9 Mike Phillips is receiving treatment for bruising and stiffness in his lower back following his man of the match performance against the Sharks last week.
Ideally, the coach wouldn't want Ellis to back up from Saturday after playing for the full 80 minutes, especially as he was woke up suffering from a mild chest infection the morning after the match. So it is vital that Blair, who hasn't started a match since the opening game against the Royal XV, comes good for Tuesday night in Port Elizabeth.
"Ian has given the medical team until Tuesday morning to work on Mike and he will have a full fitness test then. We've screened all three scrum halves and we are happy they will all be available for selection later in the week," said Lions doctor Gary O'Driscoll.
"Tuesday may become a bit of an issue for the selectors and by giving us the extra time to work on Mike's ankle shows they have some concerns."
As well as Williams to turn to in a crisis, the Lions could always ask Riki Flutey to revert back to his half-back days. He played for New Zealand Under 19 as a scrum half before moving into the midfield.
Other than the concerns over Blair, the Lions still seem to be winning on the injury front. Rob Kearney's dead leg against Western Province will mean he doesn't play on Tuesday while Martyn Williams had to have five stitches put into a cut in his knee.
His troublesome shoulder stood up well to only his second outing on tour and he will be fit to play at the weekend if selected. So, too, will his Cardiff Blues team mate Jamie Roberts, who suffered a stinger to his shoulder in the win over the Sharks.
"We are amazed we have got this far on tour with so few major injury worries. The average number of players sent home from recent Lions tours has been six or seven and so far we've only lost Stephen Ferris and Leigh Halfpenny," said O'Driscoll.