The initial impact made by the midfielder following his move from Dundee United in February 2015 steadily diminished as his performance levels dipped while deployed in a wide left role.
But Armstrong has suddenly become a central figure for the Scottish champions again as he savours being allowed to operate in his favoured position in the middle of the pitch under manager Brendan Rodgers.
The 24-year-old admits life under Rodgers’ predecessor Ronny Deila had become something of a trial for him as he found himself at odds with the Norwegian coach’s opinion of where he should fit into the side.
“That was a developing frustration for me last season, playing on the left,” said Armstrong. “But this season under the new gaffer, after initially starting off on the left, we had conversations that left us a clear picture in my mind and his mind that my best position is in the middle.
“I think it was a conversation where we both expressed different things and I expressed that my desire was to play in the middle. I’d had enough of the left and I’d like the chance to stake my claim for the middle and I’ve been working hard since to try to force my way in. I see my season so far as one of a learning curve, working hard in training to try to force my way in. Obviously, playing in the middle is a lot more comfortable for me. That’s where I feel I’m best and it’s good to know that, when I do play, I will be playing in there and that gives me confidence.
“I was playing last season and that was the most important thing, but I knew I could play a lot better in the middle. But I think he [Deila] liked other individuals better in the middle, I don’t know. I don’t want to talk about it too much now. It’s gone, it’s finished.
“There’s always uncertainty when a new manager comes in, whether you will adapt into their plans, but it’s obviously been very nice to hear recently that he [Rodgers] likes the way I play. If I can adapt into his style and help the team then that’s very good for me. There were a few fine details I had to learn after speaking to him. I had to step back and get in the queue for that central role and work hard in training to try and prove myself and prove that I could play in a No 10 role or a No 8 role. That was hard work and a challenge for me. But I have been keeping my head down and trying to prove myself.”
Despite his difficulties under Deila, Armstrong insists he never considered seeking a move away from Celtic.
“No, I wanted to play in central midfield and I wanted to play here,” he said. “There were a lot of elements of frustration that came with playing on the left-hand side. Obviously, you are judged in every single game and on a bigger scale here. If your performances aren’t up to scratch you know about it. It was a frustrating time playing on the left-hand side. But, as I say, now that has gone. This season I am playing in the middle and it is up to me to prove to myself and to the manager that I am good enough to be in there.”
Armstrong’s form has been sufficient to force himself ahead of Nir Bitton for one of the central midfield positions in recent weeks, culminating in the Scot making his first Champions League group stage starting appearance in Tuesday’s 1-1 draw against Borussia Monchengladbach in Germany.
“It was a nice experience to play from the start against a good side,” reflected Armstrong. “Our performance was much better than the home one against Borussia and I would like the opportunity to play in more Champions League games. It’s different with the European teams’ style of play. They like possession football and so do we. The likelihood is that, if you give the ball away, they are more likely to punish you than domestically, with all due respect to the teams here. They have quality all over the field.
“You always want to challenge yourself against the best and we certainly have a lot of the best players in our Champions League group. That’s why it’s been great experience. Obviously we want to pick up points and had a very good result against Manchester City, but in that game and on Tuesday night we could have maybe taken a bit more.”
Armstrong had been tipped as a potential inclusion in the Scotland squad for next week’s World Cup qualifier against England at Wembley but missed out again when Gordon Strachan named his party on Wednesday. But the former under-21 international is happy to remain patient.
“If I can become a regular in the Celtic team and put in good performances on a consistent basis then it would stake a good claim for Scotland,” he said. “But I have always said my focus is on playing well here. The rest follows.”