The winger was used sparingly by Deila last season, as the Norwegian boss tried to help him overcome his injury problems, and his tactic to ease the Scotland international back to fitness seems to have worked.
Forrest has already made 13 appearances for club and country this term, with his performances being of a consistently high standard.
And Deila said: "I am so happy for him. I have talked a lot about him since I came here and I feel that there has been more talk than there has been performances from him because of his injuries.
“But now I think you can see what is in him – he is fitter and stronger and getting better all the time. This is just the start of it for him.
“He is training with us consistently and he playing games and that is exciting. He has so much talent. The more he plays then the more we will see what he is capable of but I am very happy with the way he has started the season."
Forrest's fitness issues have been a problem for Celtic, as they were clearly stopping the youngster fulfilling his potential.
The 24-year-old has shown his ability in flashes throughout his career to date, but has rarely managed to take in a prolonged run of games.
As a result, his form has been patchy, but there are signs that he could be starting to develop into the star that everyone at Parkhead hoped he would be when he first burst onto the scene.
Forrest's next appearance could come on Thursday night, when Celtic take on Fenerbahce in Europa League action.