CALLUM McGregor spoke last night of his disappointment about being overlooked by Gordon Strachan for Scotland's World Cup qualifier against the Auld Enemy at Hampden.

But as the 23-year-old midfielder soaks up the sun, surrounded by family and friends, in the millionaires' paradise of Puerto Banus for the next ten days, perhaps reviewing his break-out season as a Celtic player and the starring role he played in the club's unprecedented invincible treble, it is hard to imagine that he will be too inconsolable.

"I've not spoken to the manager [Gordon Strachan] and I have booked a holiday so unless I get a call I reckon that is my season over," said McGregor, left out of the 29-man group which will be cut to 23 this week. "Of course it is disappointing not to get the call. But the manager picks the squad that he thinks can do the job and that is fine by me. I just have to focus on my Celtic stuff, getting days like this, and going on to be a treble invincible."

Read more: Patrick Roberts - Being a Celtic invincible treble winner will always be the highlight of my careerHeraldScotland: GLASGOW, SCOTLAND - MAY 27: Scott Sinclair and Callum McGregor of Celtic  celebrate during the William Hill Scottish Cup Final between Celtic and Aberdeen at Hampden Park on May 27, 2017 in Glasgow, Scotland. (Photo by Ian MacNicol/Getty Images).

It wasn't so long ago that the notion of McGregor becoming a Parkhead first-team regular, let alone an international, appeared far fetched. Now, there are howls of anguish amongst the Tartan Army at a form player like him being controversially omitted from a Scotland squad. The player himself now accepts that, if he keeps improving at his current trajectory under Brendan Rodgers - no mean feat given the likelihood of increased competition next season - international honours should only be a matter of time.

"Obviously I have had a great month or so and scored some big goals as well and that gets people talking," said McGregor. "It is up to me to keep working hard - I will go away and get a few weeks off then come back ready to go again. You have just got to keep knuckling down and showing what you can do. Then if the call comes then great."

Now 23, McGregor has been attached to the club since the age of nine and this is the first treble he has seen, not just the first invincible one in the long history of professional football in this country. The sum total of his contribution to the campaign was 46 appearances, totalling 2,769 minutes, seven goals and 11 assists, but his tidy possession work and ability to travel with the ball has been a real feature of the season. His day on Saturday involved getting one of those assists for Stuart Armstrong's equaliser before being unceremoniously shunted to left back, a position where he didn't appear hugely comfortable. For a moment - when he was robbed by Jonny Hayes only for a miscommunication to deprive Kenny McLean of a tap-in for 2-1 - he almost went down as the man who cost Celtic the invincible treble.

Read more: Patrick Roberts - Being a Celtic invincible treble winner will always be the highlight of my career

"It was a pretty scary moment," conceded McGregor. "Normally when you make a mistake like that you get punished for it but thankfully this time we didn't. It is hard to explain that one - you just think you have more time than obviously you have got. But to be fair I think that was one of the only breakaways they had in the second half."

Celtic had a lot to lose on Saturday, particularly when they went a goal behind so early, but rather than be freaked out by it all, McGregor said that the players had derived inspiration from the 50th anniversary of the Lisbon Lions. "I think it was inspirational," said McGregor. "It has been an emotional week for the club with obviously the show on Thursday night. I think it was in the players' minds that we had to go and give the fans another night like that so to go and do it is special. To go and become the treble invincibles is an incredible achievement. You are writing yourself into the history of Celtic Football Club and you know how big that history is.

"But the boys just kept coming into training every day, enjoying it and working hard for it, we just seemed to take it in our stride," he added. "There didn't seem to be any big panic about it. The manager was clear as well before the game, that if we didn't get it then relax, it was fine, don't panic about it. He was very good at keeping the boys calm."

The only problem for McGregor and the rest of this Celtic team is how to keep improving next year. Making it out of the Champions League group stages - rather than just making it in there - is one goal to aim at. "It [an invincible treble] is quite hard to beat it, isn't it!" McGregor admitted. "But if we keep that hunger and willingness to work hard for each other and work hard in training, the manager has got so many things he can teach us. As long as we stay a tight group and everybody trusts each other then I am sure we can take the next step. Obviously we want to get into the [Champions League] group stages again, and hopefully go one better. But we know just how hard it is to get there in the first place."