The British rower was cycling through Arizona filming a new show for the Discovery Channel when he was involved in a collision with a truck.
He is currently semi-conscious in a neuro-trauma ward and will need six months to recover from his injuries, according to his wife Beverly Turner.
She tells Britain's Daily Telegraph, "I'd spent years worrying that James would plunge down a crevasse in some remote wilderness, but it was a road traffic accident in the U.S. that has left him with a fractured skull and damage to his brain."
Turner reveals Cracknell has had bleeding and swelling on his brain, which could cause personality and motivation issues - but she insists his prognosis is good.
She adds, "Only those closest to him may be able to tell a subtle difference."
The pair shares two children, Croyde, six, and Kiki, 16 months.