The RSPCA had planned to wait until the end of the winter breeding season before trying to catch the seal at Horsey beach.
However, he has not been seen since late December.
Alison Charles, RSPCA hospital manager, said: "We will just never know now if it's a happy or sad ending."
It was believed the bull seal's deep wound was caused by netting.
Experts said it had been too risky to try catching him for treatment when he was surrounded by females until the end of the breeding season in late January.
Any action could also have disturbed mothers with their pups.
If he was darted, there was a danger he could bolt into the sea, lose consciousness and drown.
Peter Ansell, from the Friends of Horsey Seals group which monitors the colony, said: "He might be dead or he might have swum off to pastures new.
"It's highly unlikely we will see him again - it will remain a mystery."
The RSPCA had assembled a team and equipment at its East Winch centre near King's Lynn to try to catch their first wounded seal.
Ms Charles said: "I recently went to inspect two females who also had been seen with netting around their necks, but it had fallen off.
"Their wounds were healing and it seems it may have been possible for them to use their flippers and nails to it.
"The bull seal's wounds could also have healed."
Volunteers counted 1,018 pups at Horsey this winter - a record for the colony since seals began using the public beach north of Great Yarmouth in 2003.
A seal spotted on a Norfolk beach with a wound around its neck has vanished and its fate is likely to "remain a mystery", wildlife experts have said.