Celtic loaned Liam Henderson out to Rosenborg at the start of March.
Henderson could have been forgiven for feeling slightly frustrated at the Bhoys’ purchase of Armstrong from Dundee United in January, as it blocks his route to the first-team, but the 19-year-old has decided to view it as a positive.
He believes that added competition should be welcomed at Parkhead and wants to learn off the very talented Armstrong.
Henderson said: "It's great that Celtic sign these players. Stuart, especially, is a similar position to me and in training all I look to do is learn from him. I'm just a sponge looking to absorb it all and Celtic signing these players is good because it expands my learning."
Henderson was only able to train with Armstrong for a brief period before being sent out on loan to Rosenborg.
The youngster was encouraged to get first-team football by Ronny Deila and has been playing regularly for Rosenborg since March.
During that time he has impressed in Norway, while Armstrong has gone on to cement his place in Celtic's starting line-up.
Henderson will be looking to snatch that place off his current mentor next season, though, as he looks to find competitive minutes at Parkhead and convince Deila of his worth.