Last month, Lomu was revealed as one of 12 sports stars who would step into the ring at the charity event Fight For Life in December.
Organiser David Higgins said a colleague told him Lomu had a possible health scare but he could not confirm details.
"If there is an issue, obviously first and foremost, we are very concerned for Jonah and his family. In terms of our event, it's not really an important thing compared to his health," Higgins said.
All Blacks manager Darren Shand said he had not been informed about Lomu's condition and was not planning to tell the players before last night's game against France.
University of Otago's head of medicine professor Robert Walker said it was unlikely a high impact sport such as boxing would impact on health.
"It wouldn't have any effect at all in most cases. This guy is super fit, he exercises very well and credit to him. It's exactly what he should be doing to get back a normal life," Walker said.
"As fit as he is, with all the muscles he has developed to protect the rest of the organs, they would protect the kidneys as well."
He said two major complications which transplant recipients could face included infections and cancer. "There's an increased risk of infections because your immune system has been modified, and there is always a risk of cancers.
"People with chronic kidney disease, whether they have a transplant which is working well or they are on dialysis or have chronic kidney disease, always have a higher risk of cardiovascular disease ... and that is still the biggest thing that kills our people."
Lomu appeared at the Rugby World Cup opening ceremony as the hero of rugby for players like 13-year-old Ethan Jack Bai.
Last night Ethan Jack's dad Brenton said Lomu was in good shape during the opening ceremony.
"EJ hung with him quite a bit before the opening ceremony. He danced with the boys and went through the ceremony a couple of times in practice.
"He was animated and relaxed. He really enjoyed it and I was taken by him by how able he was and approachable," Bai said.
Lomu and his wife Nadene celebrated their son Dhyreille Semisi Kiole Lomu's first birthday just two days before the opening ceremony.
"Happy, happy birthday my big baby boy Dhyreille, time to bounce, eat loads, eat some more and have fun with family and friends..love you so much my heavyweight!!," Nadene wrote on Facebook.
