The Celtic pair were allowed to play against former side Dundee United in the Scottish Premiership on Saturday afternoon, having been cup-tied for the club's previous meetings with Jackie McNamara's men.
And it was Mackay-Steven who opened the scoring for the Bhoys as they beat United 3-0 in the Scottish Premiership.
John Guidetti then added a second before Denayer himself completed the scoring.
But the 19-year-old knows the importance of the former Dundee United duo, heaping praise on the impact they've made since moving to Parkhead. He certainly isn't the first to do so...
JD: "It's important to play with players like this on your side and I think it's good for us to have players like that."— Celtic Football Club (@celticfc) March 21, 2015
Celtic boss Ronny Deila, however, wants more from Mackay-Steven and Armstrong next time, who he feels were distracted by the 'special' occasion of facing their former club.
Following Aberdeen's 1-1 draw at Dundee, Celtic are now five points clear at the top of the league - and still have a game in hand over the Dons.
The 'T' word is over-used when it comes to Deila's first season with the Hoops. But it's certainly on the agenda...