Bimbo, 28, was found with a deep wound in his side by a local resident in Eltham, where he was being kept after retiring in 2012.
Lucy Morris, who lives nearby and found the injured animal, said Bimbo was seen by a vet who confirmed it was a stab wound.
The oldest in the herd, Bimbo was said to be in a state of shock and had contracted laminitis, a disease similar to thrombosis in humans.
In an update on her crowdfunder page, which has raised more than £2,000 towards the vet bills, Ms Morris said Bimbo had "perked up" and was "showing some of his bolshy character again".
The donkey is one of several owned by Lorayne Ahmet, whose father ran donkey rides near Greenwich Park for more than 60 years before he died in 2012.
According to local newspaper the Newsshopper, Bimbo and another donkey managed to escape from their enclosure.
Ms Morris was able to return them to the field but noticed Bimbo was wounded.
"That's when I saw what I thought was a kick or gash oozing from Bimbo's side," she told the paper.
Ms Ahmet added: "I transported the donkey to Sevenoaks for a vet to attend who confirmed my fears.
"It was a stab wound. I can’t believe some one could be so cruel to do this.
“Most of the donkeys are quite shy, but Bimbo is domesticated so I think that’s why they went for him.
“There have been a few stabbings in Eltham of late and I have seen youths messing around with the donkeys. I think one of them could have stabbed Bimbo as a gang initiation task."
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