The American suffered brain damage and other injuries - including fractures - after he was hit by a auto while out cycling with friends in Italy on Wednesday.
The update was on The Express newspaper website.
He was stabilised at a nearby hospital before he was transferred to Maurizio Bufailini Hospital in Cesena.
The American rider, who now races in the World Suberbike Series, was cycling along the Rimini coastline when he collided with a auto and suffered serious head and chest injuries and has reportedly been place in an induced coma at the Bufalini hospital in Cesena.
From the past season, he returned to the Superbike Championship with the Red Bull World Superbike Honda, replacing Sylvain Guintoli alongside Michel van der Mark.
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His Honda team said that as well as having his fiancée Jackie by his side, Hayden has now been joined by his brother Tommy and mother Rose who arrived in Italy from the United States earlier Thursday. Italian Media said that Hayden had suffered a cerebral edema and lower body fractures and surgery had not been possible immediately.
"After a few years we fought for the title down to the last race in Valencia and, unfortunately for me, he won and became MotoGP world champion".
Hayden competed in the Italian round of the Superbike World Championship at Imola last weekend.
"Come on Nicky, we're all with you", added the 38-year-old. That was reported by Reuters.
Meanwhile Valentino Rossi, who was beaten to the 2006 MotoGP world title by Hayden in dramatic circumstances, posted an emotional tribute to the American.
"The most handsome thing I have about him is when he gave his hand to me after the unlucky race of Valencia 2015, on the lap of honour".