Philip Clowes had taken his two-year-old dog Oscar for a walk in Central Forest Park, Hanley on Saturday.
But his pet severed six tendons and a major artery in his leg while swimming in the site’s lake.
Mr Clowes believes the injuries were caused by a metal tin in the water.
Now he is warning other park-goers not to let their animals or children in the lake.
The 55-year-old said: “It’s frightening that Oscar sustained such serious injuries just from having a play in the water.
“There are no signs to stop people or animals from bathing in the lake but clearly it’s not safe.”
Mr Clowes, an electronics technician, who lives in Derek Drive, Sneyd Green, walks Oscar in Forest Park several times every weekend.
He said: “I’m usually very cautious about letting him go in the lake.
“But he loves the water and it was such a hot day I decided to let him have a splash around – he was still on a long lead.
“I saw him get tangled in a branch and I gave a gentle tug on the lead to help him out.
“Then I noticed that he was covered in blood– it was pumping out very fast.”
Mr Clowes made an emergency call to the vet and phoned his wife, Jacqueline, who arrived minutes later with towels to bandage Oscar’s leg.
The couple took him to The Croft Veterinary Clinic in Newcastle and he was on the operating table within an hour.
Oscar will be unable to walk for four weeks, and unable to do any real exercise for six months.
Mr Clowes added: “The vets were fantastic but there’s an outside chance he may not recover at all, and he’s a lively dog, it’s going to be a nightmare trying to keep him rested for six months.
“He will need medication, therapy and hydrotherapy.
“With the hot weather I would just say to anyone who wants to let their dog or child swim in that water, I wouldn’t even consider it.
“You just don’t know what’s in that water, it’s so murky.
“There’s so much rubbish there by the end of the weekend that anything could get stuck in the lake.”
Paul Breeze, chairman of the Friends of Central Forest Park, said: “We are constantly flagging up the litter problems.
“It is disappointing for us but we are trying to keep on top of it because it is an on-going situation. Yesterday a boat went out to clear up some of the rubbish.”
City council spokesman Jeremy Herbert said: “We take this issue very seriously. Our parks team do check the pond every day for any dumped materials or rubbish and they have only just cleared the lake. Unfortunately it is very difficult to spot absolutely everything hidden in the mud.
“This extra hot period has highlighted the issue so we are now erecting signs at all waters managed by the city council asking everyone to keep out of the water, and to keep their pets out too.”