Armstrong joined the Bhoys from Dundee United on Deadline Day and Deila revealed at his press conference this afternoon that the central-midfielder will be considered as a first-team player.
Celtic have been in good form recently but the Norwegian coach is in no doubt that Armstrong can improve his squad and believes the 22-year-old has a bright future at Parkhead.
RD: "I see @StuArmstrong16 as a first team player and he's going to be very important for us."— Celtic Football Club (@celticfc) February 6, 2015
Armstrong has managed to notch six times in 22 appearances already this campaign and, if he can be just as effective with the Bhoys, he looks set to be a great addition.
There are question marks over whether he will fit in at the moment, though. Scott Brown is never likely to be dropped, while Nir Bitton has been one of the side's in-form players in recent weeks.
Competition for places is never a bad thing, however, and Deila will be hoping Celtic's high level of competition will help Armstrong push on to the next level.