The flying winger has not played for six months after suffering a groin injury while training with Scotland ahead of their friendly in Poland in March.
His recovery has been hampered by a thigh strain. However, slowly but surely through the pre-season, Forrest has been inching his way back to fitness.
And skipper Scott Brown has revealed the man who scored the goal which took Celtic through against Karagandy last year is looking tremendous in training. Now Deila is seriously considering unleashing his dynamic pace against Legia as the Hoops chase the three goals they need to get through.
It would be a brave but risky strategy from the manager, who threw Jo Inge Berget straight into last week's first leg in Warsaw just 48 hours after the wide attacker signed on loan from Cardiff. That plan backfired badly as Berget failed to make any impact and was removed early in the second half.
Deila has had much longer to assess the condition of Forrest. And, when asked if he has any chance of making his comeback in this game, revealed: "Yes, he can play some part."
Whichever players are selected for this retrieval mission, which, if he pulls it off, Deila admits will be "one of the greatest nights of my life," the final instructions he will give them before they run out at Murrayfield will be: Enjoy yourself - and let there be no excuses.
He insisted lessons have been learned from the 4-1 defeat, but does not plan to change much from the way he will ask his men to play.
Defending much better and not conceding possession in dangerous areas will be the main improvements he demands.
An early goal would fire up the Hoops support, and test the nerve of the Polish champions.
But Deila - who insisted he knew nothing of a reported bid from West Brom for Kris Commons - explained, while his side will attack, they can't go gung-ho. He said: "It's important to be patient because we can't go straight into attack without defending. It always starts with good defending."
"If you look at the goal we scored in Warsaw, it was from good defending. We won the ball in a good area and went straight into attack and scored."
His side went on to take a battering in the first leg, but Deila has not been spooked and remains philosophical. He said: "To lose is a disaster - but it is a bigger disaster here.
"I understand this, but I have to manage that. I can't think about disaster.
"I've been thinking a lot, and it has been important to get a lot of energy into the game because that was the most disappointing thing for me (in Warsaw).
"I've heard about the Celtic Heart. That's a fantastic expression. That's what was lacking in Warsaw and it's what I want to see tonight.
"We have everything to gain if we can turn this around."