The 35-year-old American, who won the MotoGP championship in 2006, was cycling on a road along the Rimini coastline on Wednesday afternoon when a road car collided with his bicycle.
The Red Bull Honda rider received some medical attention before being transferred to a nearby hospital for treatment.
According to reports, the rider remains in a serious condition, with bleeding on the brain and a broken leg and pelvis, with doctors saying he is still too weak to be operated on.
Honda WSBK tweeted yesterday: “Thank you to all Nicky’s fans for your support after his accident. He’s receiving medical attention in Italy, and we must now be patient.
“The medical teams are working hard, and we will provide updates as soon as they are available.”
Hayden currently sits 13th in the Superbike World Championship standings after a 12th-place finish at the Motul Italian Round at Imola on Sunday.