MANY different factors have led to Stuart Armstrong, after a difficult year and a half, establishing himself at Celtic this season and forcing his way into contention for a Scotland place. Brendan Rodgers taking over from Ronny Deila as manager at Parkhead is a significant one. Being moved from a wide left berth he looked decidedly uncomfortable in to his favoured position in the centre of the park is another.

But Gordon Strachan - who was never, despite his distinct lack of height and physical presence, have accused of lacking belligerence during his own distinguished playing career – believes that developing a nasty streak has been instrumental in Armstrong finally fulfilling the enormous potential he showed when he was emerging at Dundee United.

After naming the player who scored the opening goal in the Ladbrokes Premiership match against Rangers at Parkhead on Sunday in his 30-man squad for the double header against Canada and Slovenia next week yesterday, the national coach recalled an encounter with the midfielder a few years ago that concerned him.

“My wife and I went to a dinner and he made an acceptance speech,” said Strachan. “We all went ‘what a lovely lad he is’. But I’m afraid you have to turn into a devil on the pitch. You don’t have to be a devil, but there has to be a devil inside you.

“I think that personality has grown because he joined Celtic and had to deal with what comes with that. He was just a lovely fellow before. 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. Stuart had five shots on Sunday and all five were on target. That’s good going.

“His personality has grown. The fact is that since he has been with Celtic 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. But that has improved his character. His character has been built up and he has been a big personality on the pitch."

Armstrong is unlikely to be considered for the friendly against Canada at Easter Road a week tomorrow due to the fact that Celtic play Dundee at Dens Park this Sunday. The other uncapped players in the squad, Tom Cairney of Fulham and Ryan Fraser of Bournemouth, are far more likely to feature.

But he has a decent chance of starting alongside his club mate and captain Scott Brown in central midfield in a Russia 2018 qualifier with Slovenia at Hampden on Sunday week which Scotland have to win to keep their slender chances of qualifying alive and, you would have to suspect, to keep their manager in his job.

His impressive strike rate in the 2016/17 campaign – his goal at the weekend was his 11th in all competitions – has increased his chances of featuring.

“He has moved position,” said Strachan. “You have got to get right because the position that he was in previously was wide in a three at times. That has changed. 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.”

Strachan certainly has an abundance of quality central midfielders – Barry Bannan, Darren Fletcher, James McArthur, John McGinn, James Morrison and Charlie Mulgrew have all played there in the past – and the same is true of left backs.

Andrew Robertson, Kieran Tierney and Lee Wallace are all included in the squad while Ikechi Anya - who played in the 3-0 defeat to England at Wembley back in November – is the only specialist right back and even he is considered more of a winger.

Strachan, though, conceded that Robertson, Tierney and Wallace could, if required, switch sides. “I thought Ikechi did well against England, but it could easily be done,” he said. “It can definitely be done. There is no point in having a system and playing a right-back who’s not very good and having two great left-backs sitting beside you that can play right-back.”

Centre half has been another problem position for the national team in recent seasons. In the Group F match in London last year Strachan had to play Grant Hanley even though he had not been featuring for Newcastle United there and his side conceded three soft goals.

However, the availability Charlie Mulgrew, who has been playing in the heart of the Blackburn Rovers defence in the English Championship, once again could help against Slovenia. "I've seen a lot of Blackburn,” said Strachan. “Charlie was in midfield and that got his fitness up. Then he's been at centre half over the last couple of weeks. He's comfortable with that. He was injured at the weekend but hopes to be back playing against Fulham tomorrow, which is a game I will be at.”