Gary Mackay-Steven left Celtic for Aberdeen in the summer transfer window.

Armstrong played with the 27-year-old at Dundee United before the duo made the switch to Celtic in 2015.
Mackay-Steven struggled to make an impact at the Hoops and joined Scottish Premiership rivals Aberdeen in the summer transfer window for a fee reported by The Sunday Post to be worth £275,000.
The Scotland international is a very good winger who will recover his form at Aberdeen if he plays regularly.
The former Airdrie United winger is set to line up for Aberdeen against his former club Celtic at the Pittodrie Stadium in the Scottish Premiership on Wednesday evening.

Ahead of the match, Celtic midfielder Armstrong, who is an important player at Celtic Park, has spoken highly of Mackay-Steven, and has suggested that he understands why his former teammate decided to leave Celtic Park in the summer transfer window.
“I think it was an important move for Gaz to make and I’m pleased it is working out well for him,” Armstrong told The Sunday Post about Mackay-Steven.
“Having played football with him for half-a-dozen years or so, I have seen what he can do and what he is capable of.
“He is a top-quality player, but everyone wants to play football regularly and he wasn’t always getting that with us. Up at Pittodrie he certainly has a chance to do that.”

Meanwhile, David Provan has suggested in The Scottish Sun that Cristian Gamboa was a scapegoat for Celtic against Bayern Munich last week.
Gamboa’s performance against Bayern at the Alliana Arena in Munich in the Champions League was hugely disappointing.
Former Celtic winger Provan has suggested that the 27-year-old right-back’s display eclipsed the poor performance of his teammates.
“Cristian Gamboa did his team-mates a major favour in Munich,” Provan wrote in The Scottish Sun. “After Celtic froze in the Allianz arena, the Costa Rican took the rap for everyone.
“On a night Gamboa was needlessly exposed to Bayern flying machine Kingsley Coman, the full-back was a convenient patsy.
“His performance was so poor, nobody was looking elsewhere for scapegoats. After a tie in which only Craig Gordon earned pass marks, Gamboa took one for his team-mates.”
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