Zoo spokeswoman Kate Orgias said Ngala's weakened physical state left him vulnerable to attacks from the zoo's other three lions.
She said the decision to euthanise Ngala was made in the knowledge his physical condition would not improve, and his overall welfare and quality of life could not be maintained.
"This type of injury is one that could also happen to a lion in the wild, and due to reduced mobility, would ultimately be fatal," Ms Orgias said.
"Unfortunately, surgery to repair the ligament -- something that rugby players sometimes undergo -- is impossible for an animal like this due to the type of after-care that would be required."
Ngala is survived by female lions Sheeka, Kura and Amira. His death leaves Auckland Zoo without any male lions.
African lion Ngala. Photo / Auckland Zoo
The zoo said there were no plans at present to import male lions, but it would be considering it as part of its normal collection planning.
Ngala came to Auckland Zoo from the Cango Wildlife Ranch in South Africa in 2003, along with fellow male Lazarus, who was transferred to a zoo in New South Wales in 2012 as part of an Australasian regional breeding programme.
Video: Ngala gets root canal
Meanwhile, Wellington Zoo said it was saddened by the death of its male golden lion tamarin Orolito.
The diminutive South American primate died unexpectedly yesterday afternoon.
Zoo spokeswoman Charlotte Whitelaw said staff were waiting for the results of a post-mortem examination to determine the cause of death.
Orolito was found to be quiet and lethargic yesterday morning, and was taken to the zoo's vet clinic for fluids, glucose, warmth and antibiotics.
The zoo said he died a few hours later, despite further care throughout the afternoon.
Orolito is survived by female tamarin Clementine.
Male golden lion tamarin Orolito. Photo / Wellington Zoo
Ms Whitelaw said tamarins were very social primates, so the zoo was in contact with the species coordinator at the golden lion tamarin breeding programme in Brazil.
Depending on her genetic make-up, she may be paired up with a breeding male or put into a general social group.
The zoo's vet team was keeping a close eye on her in the meantime.
"We're just wanting to do the best thing for her and take in account that these animals are endangered, and we want to be boosting their population numbers."
Orolito settled in at Wellington Zoo last September after he was transferred from Cleveland Zoo in Ohio, United States. Clementine came from Santa Ana Zoo in California.
