While the wing giant aims at making a comeback with Wellington in next month's National Provincial Championship to push his claims for World Cup selection, Semisi Lomu has urged his famous son to urgently reconsider.
"The World Cup is not worth his life," Mr Lomu told a New Zealand newspaper.
"He needs to turn his mind off rugby and the World Cup and focus solely on getting better. He's thinking too far ahead. He may getting pushed by people I don't know."
Lomu requires dialysis four hours a day, three days a week and a transplant as he battles the chronic complaint that has sidelined him since early this season after re-surfacing.
"Our family is extremely worried about him," Mr Lomu added, while confirming the game's biggest name, once married, has become engaged for a fourth time, to 30-year-old Aucklander Fiona Taylor.
"He's not the Jonah I know. I've seen him break into tears. Jonah is not usually like that.
"I've always dreamed of Jonah being in a World Cup winning All Blacks team. But right now I would rather see him get back to good health than see him holding the World Cup."
His father's concerns are at odds with Lomu's website comments last week.
"I feel stronger now than I did in the last World Cup," he wrote. "I am back training when I am not on dialysis and when the Wellington Lions get back to training, I will be with them."
Lomu's manager Phil Kingsley-Jones also said Lomu was on track to make his return to the game.
"He's very positive, he's looking very good and very sharp," said Kingsley-Jones. "He wants to play NPC."