Eloquent and intelligent, Armstrong was perhaps too quiet and not demonstrative enough to really take a game by the scruff of the neck, although he was undoubtedly previously hindered by playing in a wide role for Celtic.
However, Strachan, pictured, has seen a different animal – one now expressing himself and imposing himself on matches as his performance in Sunday’s Old Firm derby highlighted. The former Dundee United attacker might be ideally suited for a role in the 26 March World Cup qualifier against Slovenia where Strachan needs a win or the campaign is over.
The Scotland manager said: “I think he’s a different personality now. He’s become a personality on the football field. He has enjoyed being that personality.
“I think what he wanted to do before, and I might be wrong, is be a good team player. But he had more than that in him.
“What he is trying to do now is be the best player on the pitch first and then be a good team player. I think it was the other way about before.
“He was just a lovely fellow. Me and my wife went to a dinner and he made an acceptance speech and we all thought ‘what a lovely lad he is’ but I’m afraid we have to turn into devils on the pitch. You don’t have to be a devil but there has to be a devil inside you and I think that personality has grown because he joined Celtic and had to deal with what comes with that.
“It has not gone all that smoothly. It hasn’t been a great time for him. He has had to take a few knocks, get on with it, come back. That has improved his character. His character has been built up and he has been a big personality on the pitch.”
The 25-year-old has scored 11 goals this seasons, including Sunday’s strike against Rangers, and is probably the most improved Celtic player since Brendan Rodgers replaced Ronny Deila.
Strachan said: “He has moved position. He has gone to that central midfield position which I always thought was his best.
“He has an engine that means he runs into positions and picks the ball up. Some people stand in positions, he runs into positions.
“He runs beyond strikers as well, which is good. He has got the taste for goals now which is why he has come to the fore. When you score goals you get recognised.
“Stuart had five shots on Sunday and all five were on target and that’s good going.”
Strachan believes the rest of his Celtic contingent – Craig Gordon, Kieran Tierney, Scott Brown, James Forrest and Leigh Griffiths – have also benefitted from the Rodgers effect.
He will not use any of them in the Canada friendly on 22 March as Celtic are in action three days earlier in Dundee when they could clinch the title if Aberdeen lose to Hearts the day before.
The one area of the pitch where he appears to have fewer options is right back but he hinted that he could convert one of this three excellent left backs –Tierney, Andy Robertson and Lee Wallace, pictured right.
Ikechi Anya played the role in the last Scotland match – the 3-0 defeat by England at Wembley – but Strachan says he will not be hamstrung by left-sided players operating at right back.
He said: “Lee Wallace was terrific against England, he was absolutely terrific and he was terrific on Sunday as well. He kept Rangers going and I thought he was first class.
“It could easily be done. It can definitely be done. There is no point in having a system and playing a right-back that’s not very good and having two great left-backs sitting beside you that can play right-back.
“Nobody bats an eyelid when it goes the other way, so it can be done.
“But I thought Ikechi did well against England, nothing really affected him. Pace wise, he’s good. He can head the ball and get forward.”
Strachan has had four months to mull over the Wembley defeat and only a win over Slovenia will mean that the return with England on 10 June will have any real meaning.
He added: “You can screw yourself up but if you look at it, two minutes changed the outcome of the game.
“You have to put that behind you and look forward and get on with it. And we’ve not left the squad the same.
“New people have come to the fore, who were not leaving footprints before the England game but they are leaving footprints now.
“Because of that I would have no qualms about putting them in.
“If we can get the players into the positions against Slovenia and England and finish off these moves then the world is a different place.”