The Celtic boss wants to unlock the 23-year-old's 'special potential.'
The 23-year-old has been criticised by Celtic supporters for a mixture of injury-enforced absences and poor performances this season.
But Deila has rushed to Forrest's defence, suggesting he has 'special potential' and that's why the club are working so hard on getting the best out of him under a different training regime.
Indeed, the Bhoys boss can understand fan frustration with the winger but says he can be a success in the longer term - and can still make an impact this campaign.
He told reporters: “James can still make an impact in this campaign as well. I can understand the fans’ frustration with him. On that pitch you have to perform, that’s the demands of Celtic.
“But we have to look longer term and see what’s happening in the background. We know there is special potential there. That’s why we’ve been working differently in his training and treatment."
Deila also reserved praise for Forrest's current physical condition, with the Norwegian appreciating the extent of the Hoops star's previous injury problems.
The 23-year-old hasn't pulled a muscle in five months, according to Deila, and can return to his best if he keeps working along the same trajectory.
The Celtic boss added: “James hasn’t pulled a muscle now in five months and that’s a big step forward for him. He has been unlucky and needs time to get back to his highest level.
“I have a long-term plan with James because his history is not good. In fact, it’s very bad injury-wise. But he’s in a better place now. I’m thinking, next season he’ll be on top again."
Deila, of course, was speaking ahead of Sunday afternoon's Scottish Cup quarter-final clash at Dundee United, which Forrest started - and received plenty of criticism for his display.