Taylor Phinney (BMC Racing) broke his left leg in two places as a result of a crash during the USA Cycling Professional Road Championships on Sunday, the day after winning the US time trial title.
The injury means that Phinney will now miss his debut Tour de France in July, when the race starts in Yorkshire.
Phinney crashed on the descent of Lookout Mountain after 43 kilometres of the race, breaking his lower left leg in two places and injuring his knee.
The 23-year-old underwent surgery in Chattanooga on Monday night.
Phinney had been enjoying a successful season, with an overall win in the Tour of Dubai and a stage victory in the Tour of California – as well as clinching the US time trial title.
Late attack nets Taylor Phinney the stage victory in Santa Barbara and Bradley WIggins maintains overall lead