The humane society says bear biology experts say Makoon's (mah-COON's) survival rate would be as low as five per cent.
The society says the provincial government should send the young bruin to a rehabilitation centre.
It also says Manitoba Conservation's Jim Duncan has indicated the cub will be released within the next week or two.
A ministry spokeswoman says no decision has been made.
The cub was found abandoned near the side of the road earlier this year and taken home by a man, who cared for him until the bear was seized by conservation officials.
“It is cruel and unimaginable the province is considering such a callous act for a cub who cannot adequately fend for himself," Bill McDonald, the humane society's CEO, said in a release Thursday.
The society spoke to bear expert John Beecham, who said it's best to release cubs when they are large enough and old enough to defend themselves from predators. That's usually at 17 or 18 months of age, he said.
The Winnipeg Humane Society is the latest group to join the debate about what should happen to Makoon.
About 50 protesters gathered outside the Assiniboine Park Zoo on Wednesday to rally in favour of sending the small bear to a sanctuary. Makoon has been at the zoo since he was seized.
Judy Stearns, who started an online group dedicated to stopping the bear from being released into the wild, said more than 10,000 people have signed an online petition protesting the bear's release.
She said a bear sanctuary in Ontario has offered to care for Makoon.
Duncan said the youngster is thriving and has gained weight and is nearly ready to return to the wild. He has said the bear has a 50-50 chance of surviving.
Conservation Minister Gord Mackintosh has said he will take his advice from experts in his department.
The province has said it can't send Makoon to another province over fears of spreading disease. Officials have also said Winnipeg's zoo is not equipped to handle adult black bears and many other zoos already have enough because they are a common animal.
(The Canadian Press, Winnipeg Free Press)