The Norwegian teenager will arrive in the summer.
Celtic fans may not be entirely impressed with manager Ronny Deila at the moment but there is no doubt that his most recent contact with the Norwegian game has seen him bring in an exceptional talent for the club's future.
Celtic have snapped up talented Norwegian youngster Kristoffer Ajer from IK Start. The teenager will join the club at the end of the season with Celtic beating plenty of top opposition for his signature.
The Bhoys are clearly planning for the future with this arrival and Celtic fans will rightly be excited about the future with Ajer in midfield.
Now a current member of the Celtic midfield brigade has had his say.
"New players come in and bring competitive edge, so think it will be a good addition.
"There are a lot of good players here and people fighting to get into the starting XI. There is very good strength in depth."
Armstrong himself has struggled for regular form since arriving from Dundee United last year and the last thing he needs is extra competition for places.
He will need to be at his best form for the rest of the season to ensure he is in the best position to be a first-team regular when Ajer arrives at Celtic.