Rogic has been handed regular first-team action by the Bhoys this season, and Deila has been impressed by the Australia international’s performances.
However, the Norwegian manager believes that Rogic’s inability to last the full 90 minutes of a match is stopping him from fulfilling his potential at the moment.
"Tom improves all the time. I am really happy with him. He works very hard in training and you can see that being transferred into games," Deila said.
"But he still needs to last a game, a whole 90 minutes There have been too many times when he gets tired. It has been difficult for Tom to feel as good late on in game as he does at the start.
"He has missed a lot of football, which we can't forget. So he needs more games. And once this fitness comes, he will be an ever better player and he is at the top level right now."
Rogic had a difficult first two years at Celtic, with injury blocking his progress into their senior side.
It may be that his current fitness issues stem from a lack of action in recent years, and the 22-year-old will be keen to stay off the treatment table for a prolonged period of time now.
Rogic has played 17 times for Celtic and scored five times this season, with his performances establishing him as a key player in Deila’s line-up.
Rogic’s next chance to impress for Celtic is likely to come at the weekend, when his side take on Kilmarnock looking to extend their lead at the top of the Scottish Premiership table.