The 28-year-old has already been ruled out for the rest of the season after being rushed to the Princess Alexandra Eye Pavilion after the ball smacked into his face.
But now he's anxiously awaiting news on possible surgery having been ordered by medical staff to rest at home for a week.
Today boss John Hughes said: "Graham took a sore one, the ball hit him from really close up and although there was not a cut you could see his eye swell up with blood.
"He didn't have any vision out of it and we got him off to the eye pavilion instantly and his retina is full of blood. We should find out later today whether he needs an operation to release the pressure."
Stack's injury resulted in a recall for Graeme Smith for yesterday's SPL clash with Rangers, the Ibrox side crowned champions after Kyle Lafferty claimed the only goal of the game while Yves Ma-Kalambay, who has been on compassionate leave following the death of his father, was asked to take a seat on the bench.
Hughes admitted the big Belgian wasn't in "a great frame of mind," but with January signing Mark Brown also out for the season having flaked a bone in his wrist, Ma-Kalambay took his place among the substitutes.
Stack's injury is the latest blow to hit Hibs with Moroccan playmaker Merouane Zemmama facing the gruelling recovery from cruciate ligament surgery and skipper Chris Hogg forced out of yesterday's game at half-time as the back problem which has plagued him for weeks flared up again.
The events of the weekend left Hibs in fifth place, Motherwell having leapfrogged them with their victory over Hearts but, Hughes insisted, there were further signs even in defeat that his side are recapturing their form of earlier in the season.
He said: "It's been like that the last three weeks, just as at Celtic Park we were disappointed not to take something from the game."
The East of Scotland Shield will be played between the Under-17 sides of Hibs, the holders, and Hearts at Easter Road next Monday, kick-off 6pm. Admission will be 5 (adults) and 2 (concessions).