But since then he - and Scotland - have had to. Armstrong ended last year's World Cup at the peak of his powers, was then cut down by a torn knee ligament, and the pragmatic Scots found an alternative of different stamp but similar exceptional ability in Andy Nicol. The irreplaceable had, after all, been replaced.
In the circumstances this was like a gift from heaven for McGeechan, but now Armstrong is back and the coach will eventually have to make up his mind - not just for Scotland but for the Lions too. Meanwhile, as the Scots have no autumn international Armstrong, 26 next Wednesday, has all the time he needs.
This afternoon, when he plays for Jed-Forest at Stirling County on the opening day of the McEwan's League season, he returns to the national stage after an impressive build-up during which Jed have won four Border League fixtures. However tentative Armstrong may feel, the expectation of others is uncomfortably high.
'I'm not having any bother with my knee; I've taken plenty of knocks on it without any problem and my form has been as good as I could have hoped for,' he said. 'But I think people expect far more than I can give them. I'm getting fitter and fitter but I'm not fully fit yet and people have to realise that. The club do, and they know that I have to take things at my own pace. As long as people aren't too disappointed . . .'
In any event, it could not compare with Armstrong's own wrenching disappointment when the injury happened. Many observers had considered him the best scrum-half of the World Cup and when he suffered a three-quarter tear of the left medial ligament playing club rugby a week before the Calcutta Cup match against England last January, his entire career was in jeopardy.
This was to reckon without the player's persistence and fortitude, qualities he has never failed to demonstrate on the field. The knee was in a brace for six weeks, Armstrong was on crutches for seven and off his work as a lorry driver for two months. While Nicol was establishing himself in his stead in the Scotland team, Armstrong was either limping or resting and after one exploratory appearance in a charity match in Orkney he spent the summer in deliberate repose.
'There wasn't much point in training early, not with a long season ahead,' he said. 'I'm getting back to normal now but I'm not in any rush. It's far better that I build up gradually. Even though I've felt really fit in training, it's not until you get the bumps and bruises of a match that you really know what condition you're in. It's games I need. I hope to peak in January.'
As January is when Scotland kick off the Five Nations' Championship against Ireland at Murrayfield, Armstrong has evidently plotted his comeback precisely. He shows scarcely any regret at his enforced lay-off, preferring to regard it as a welcome opportunity for self-renewal.
'It was obviously frustrating being at home and having to watch Scotland play on the TV but to be honest I enjoyed the break,' he said. 'What with my various commitments for Scotland, the Lions and the World Cup, I'd been training solidly ever since 1988. Everyone needs to rest some time and this has completely renewed my enthusiasm.
'All I can do now is go out and play 100 per cent. Andy is a young lad who came in and acquitted himself well and there's no doubt I'll have a battle on my hands. But that's a good thing for Scottish rugby, for him and for me. Any player is better off being pushed or pushing someone else. It's always a fight to try to get back in but then it's already been a fight just to get fit.'
Ultimately, McGeechan and his co-selectors will have to resolve their dilemma. Armstrong was a Lion under McGeechan in Australia in 1989, a Scotland Grand Slammer in 1990, and his status in Scotland is akin to Terry Holmes's in Wales when he was an international.
But not only is Nicol the man in possession, he also followed last season's Five Nations success by impressing in distinguished company for the world team who marked the New Zealand RFU centenary - and, as McGeechan coached that side, the achievement will not have gone unnoticed.
Nicol has now reached the First Division with Dundee High and will have a point of his own to make today against Heriot's. But when the choice is made, we must expect it to be Armstrong - provided he is fit and well - and then he will have the incentive of next summer's Lions tour of New Zealand.
This is certainly the expectation, albeit partisan, of Roy Laidlaw, Armstrong's distinguished scrum-half predecessor for Jed-Forest, Scotland and the Lions. 'I wouldn't say he's 100 per cent yet; he's needing to shed one or two pounds,' he said. 'He put on quite a bit of weight while he was out but he seems to be fully recovered from the injury and he's played very well for Jed over the past couple of weeks.
'Without a doubt, there's going to be a tremendous battle. Both boys will be competing for the Scotland place and probably the Lions place as well. I'm obviously slightly biased but if Gary gets back to the form he showed in the World Cup and the year before his injury, then he would still be the No 1 in Scotland - even though young Andy Nicol played very well last season and has improved since then.
'The fact that Gary has had a fairly long lay-off and is fresh because he missed such a long time has whetted his appetite and, I believe, will be good for his game. The challenge he faces from Andy is an added stimulus - and all to the good of Scottish rugby.'
If Laidlaw's forecast comes to pass, Armstrong has every chance of joining the Lions, even if nothing is further from his thoughts. 'You can always look ahead and wonder what it would be like to be with the Lions but it's not a priority, not even crossing my mind at all. I have to try to win my Scotland place back first and I want to try to win something with the club.'
Still, bearing in mind that McGeechan is Lions coach again - and that he would not want to go into any game with any other scrum-half in the world - Armstrong should start making his arrangements.
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