The third occupant is said to be in a critical condition in hospital.
The accident happened on Black Road between Mdantsane and East London and Mnguni was admitted to St Dominic's Hospital in East London for observation overnight.
He suffered cuts from the broken windscreen and was complaining of severe chest pain.
Boxing South Africa's Eastern Cape representative, Phakamile Jacobs, was at the scene of the accident soon after it had happened.
He said the bakkie that collided with Mnguni's car was pulling a trailer carrying corrugated iron.
Colonel Kobus Smith of the Cambridge police station said he could not officially confirm the accident, but he had heard about it from a traffic officer.
While being admitted to hospital, the revered former boxing trainer, who produced IBF champions Welcome "The Hawk" Ncita, Mbulelo "Slayer" Botile, Vuyani "The Beast" Bungu and Zolani "Tiger" Petelo, said: "If it was not for the fact that I was driving a big car, I would have died. I have severe chest pains. The x-rays showed no signs of severe internal damage".
- Additional reporting by Mogomotsi Selebi