Forrest's career has been curtailed by a number of injuries, the latest a thigh problem, since breaking into the side towards the end of the 2009-2010 season.
Now Deila is prepared to sanction a lengthy period on the sidelines for the 22-year-old - including three rounds of qualifiers for the Champions League - in order to discover the source of his problems.
"James is out for some weeks," he told the assembled media ahead of Thursday's Austria tour opener against Russian side FK Krasnodar. “We will then see what happens in terms of the qualifiers and will take it week to week.
“I don’t want to rush James. He has been out for such a long time that he needs time to get his fitness up as I saw in training that he didn't have the fitness required to be consistent.
"When he is fit, then he can start playing games. He is a very explosive player. When you are in and out you can push too hard to get back and end up having to start all over again.
“It’s very important that we take our time with him and get out his potential. If it continues the way of the last year or two then that will never happen because of all the injuries.
“It’s better to keep him out for a good while now to really get him fit. From there, we can get him into games. James is an exceptional talent but you need that fitness to play game after game. The medical staff know what to do with him. It’s just about getting the time to do it.”