McGeechan is now retired, well out of it, and has swiftly been proved alarmingly right. Scotland have been unable to do without their indomitable Borderer, so much so that they practically begged him to make himself available. So, however reluctantly, he returns to win his 29th cap against England at Murrayfield this afternoon.
The comeback is very much on Armstrong's terms - which might just be storing up trouble if anyone else chose to use this special case as a wider example.
Moreover, it is a strictly temporary situation; there will be no Armstrong on the close-season tour of Argentina, no Armstrong in next year's World Cup.
Not unless there is a lot more begging, a lot more time off work 'arranged' by the Scottish Rugby Union to help Armstrong into a condition commensurate with an international rugby player. These past couple of weeks have seen him driving himself to the limit in training each afternoon rather than driving his lorry.
It is an onerous, almost overwhelming responsibility but Armstrong wishes it to be known that he has not entered a phone box on Jedburgh High Street and come out wearing blue tights and a cape.
'I told the SRU that I would try and help them out but one man doesn't make a team, so I just hope that folks aren't building up their hopes too much, because I'm not Superman after all,' he said.
We could be excused for believing this was how the Scotland selectors viewed him and they have already been proved right in one important respect: the return of Armstrong has generated a modest surge of optimism which was quite unthinkable after Scotland's 29-6 defeat in Cardiff three weeks ago, let alone the All Black annihilation that preceded it.
'He is such a quality player, the type of player Scotland badly need at the moment because it's not just his own game but the effect he has on those around him that is significant,' Roy Laidlaw, Armstrong's illustrious predecessor as scrum- half for Jed-Forest and Scotland, said.
The astute Laidlaw, now the SRU's youth development officer in the Borders, is one of the few who know Armstrong well enough to understand the decision which took him out of the Scotland team at the end of last season.
Armstrong had already withdrawn from the Lions tour to New Zealand - ostensibly on fitness grounds though there was more than a suspicion that he had had enough of rugby - when he announced he no longer wished to play scrum-half.
So much for personal ambition, of which Armstrong plainly had none. The national selectors were mortified but, funnily enough, not Laidlaw.
'I wouldn't say I was drastically disappointed because actually it didn't surprise many people round here,' Laidlaw said at home in Jedburgh. 'From a Scottish point of view everyone was bemused because it seemed so strange when most of us would have given our right hand to be there.
'It goes to show how different things make different people tick. At the time he made the decision he was genuinely looking for a new challenge because the enthusiasm had gone. Many players go through something similar and try to play their way through it, but Gary stood back and decided to play in another position.'
It did not work out. At various times Armstrong played full-back, centre and outside-half, his move away from scrum-half allowing the return to Jed of Grant Farquharson, who had done the unthinkable by leaving for Gala because his path was always going to be blocked by Armstrong.
In the end Jed-Forest preceded Scotland in asking Armstrong to revert but the game which persuaded the Scotland selectors to kneel as they pleaded came when he returned to scrum-half to lead Scottish Districts against Auckland at Hawick in November. It was as if he had never played anywhere else.
How ironic, or desperate if you are a Scot, that Scotland have taken two international hidings since then with Armstrong voluntarily waiting in the wings, so to speak. 'Gary was buzzing to play and after the game he was on a high and told me he'd like to get back for Scotland,' Laidlaw said. 'Then the euphoria seemed to die and the interest was gone again.'
Armstrong himself, reticent and reclusive, is less eager to talk about these events, whether the initial departure or the subsequent return. His aversion to questioning is such that one Scottish pressman calls him 'the Princess Diana of Scottish rugby: the more he shuns publicity, the more he attracts it'.
On the other hand, something has to happen to gee up the Scots after the Wales match and Armstrong is the likeliest to generate it no matter how much he tries to broaden the responsibility. 'I'm being honest: it has to be a 15-man effort,' he insisted.
'Scotland went out against Wales, then there was a punch-up and after that it looked as though only two of our blokes had the bit between their teeth.
'Ultimately everybody has to be pulling together but it didn't seem to me that Scotland were confident of winning.
'I know the lads' spirits have been low and I want to help them get out of a hole, but it's a big challenge, isn't it?'
But not too big, for Armstrong, who is still only 27, has international experience going back to 1988 including the Lions' tour to Australia in 1989 and, most significantly, looked the part even against New Zealand when he was the Barbarians' scrum-half at the Arms Park nine weeks ago.
'I said after watching him at Hawick that the break from being scrum-half might have been the best thing that's happened to Gary, that he might be an even better player as a result,' Laidlaw said.
'I remember playing stand-off as a youngster; I wasn't too keen on it but it was a great learning experience. Playing stand-off, centre or full-back has given Gary a broader understanding of the game. He has been able to look at it from a different perspective and maybe now has a better knowledge which will do him and the team a lot of good.'
The feeling is clearly mutual, though Geoff Cooke, the England manager, expressed it slightly differently: 'I was horrified when Scotland announced their Murrayfield team with Armstrong at scrum-half.' A ghastly prospect, one fears, for an English scrum-half. . . Kyran Bracken had better beware.
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