Seven-times Tour de France winner Lance Armstrong fractured his right collarbone in a race in Spain on Monday, casting doubt on his participation in May's Giro d'Italia.
The American announced via his twitter feed (http://twitter.com/lancearmstrong) that he would be having surgery "in a couple of days" after sustaining the injury during the first stage of Spain's Vuelta Castilla y Leon.
Asked whether his participation in the Italian stage race is in jeopardy, Armstrong said: "I am very disappointed. Very. Especially for the Giro."
In comments released in a team statement, Armstrong added: "Now I feel miserable ... I hope I can tell you more about the Giro in a week."
Armstrong, 37, came out of retirement this year and is due to make his first appearance in the Giro, starting on May 9 in Venice, before riding the Tour de France in July.
"The crash has put my upcoming calendar in jeopardy but the most important thing for me right now is to get back home and rest up and begin my rehab," Armstrong said in a team statement.
Armstrong was in obvious pain as he was helped into an ambulance and taken to hospital after the incident involving a group of riders with around 20 km left of the 168.3-km leg between Paredes de Nava and Baltanas.
He was taken to the University Clinic in nearby Valladolid for treatment and was later discharged.
Astana team boss Johan Bruyneel said the fracture has been clean and the rider's recovery should be fast, although he gave no timeframe.
The rider said he had been lucky to have avoided one of the sport's most common injuries for so long and that the only thing he could do now is rest.
"This never happened in 17 years of pro cycling. That's cycling. It's nobody's fault," he said.
On his twitter feed, he added: "I'm alive! Broken clavicle [right]. Hurts like hell for now. Surgery in a couple of days."
The Vuelta was being closely scrutinised as it was the first and only time Armstrong had been due to race alongside Spanish team mate Alberto Contador before the Tour de France.
Contador, the world's best stage-race rider, won the Tour in 2007 and there has been speculation about which of the pair would establish himself as Astana's number one.
The Spaniard finished in 27th place in Monday's stage, which was won by Joaquin Sobrino of the Burgos Monumental team, and he said the accident has frustrated Astana's plans.
"It was a good opportunity to work together at a race," he said in a statement. "Now only I can support him and wish him to recover as soon as possible."
David Vitoria of Rock Racing took second place on Monday and Jose Joaquin Rojas of Caisse d'Epargne third.
"I don't know how the crash happened because it was in the middle of the peloton," stage winner Sobrino said.
"That it was Armstrong is a shame for us given all the media attention he has attracted but it was nice anyway that he was in the race today."