Celtic snapped up the pair on transfer deadline day.
The two players were perhaps the best talents left at Tannadice and had impressed in the SPL for the past couple of seasons.
However Celtic are the perennial force in Scottish football and the pair were always going to be on their way when The Bhoys interest became apparent.
It was a move that left Dundee United fans understandably miffed. They have now witnessed the likes of Ryan Gauld, Armstrong and Mackay-Steven all leave the club in the past few months.
But their chairman has made it clear that the club had no real choice but to sell the pair. Speaking to The Courier Stephen Thompson said of the sale:
“That was probably the toughest decision I have ever made at this club.
“I have made a few tough decisions in my time but that was really, really hard.
“They didn’t ask for a transfer as such.
“However, one of them (Mackay-Steven), through his agent because they do all the work, made it clear he wanted to go.
“The other one (Armstrong) had a year left but he signed that extra year on the assumption that if the right offer came in from a club he would like to go to then we wouldn’t stand in his way.
“So they didn’t ask for a transfer as such but their heads were turned and they wanted to go.
“The club was compensated for losing them but we did what we did in the long-term interest of the club."
Celtic are certainly revelling in their rivals loss. The two players have impressed since the move, with both scoring in their opening appearances and they also shone in the recent clash with Italian giants Inter Milan in the Europa League.
Unfortunately for clubs like Dundee United Celtic hold all the power in Scottish football, built up over the years thanks to multiple successes and the reputation that has followed. With Rangers also out of the question, that has become even more apparent.
So players like Mackay-Steven and Armstrong are always going to end up at Celtic if Paradise is calling.