Alaphilippe, in white with the rainbow stripe, has won back-to-back World ChampionshipsWorld champion Julian Alaphilippe suffered two broken ribs, a broken scapula and a collapsed lung after hitting a tree during Sunday's Liege-Bastogne-Liege one-day race.
The 29-year-old Frenchman's condition is stable, but he has been kept in hospital for observation, said his Quick-Step Alpha Vinyl team.
A huge crash brought down many riders 62km out, with Alaphilippe landing in a ditch and hitting a tree.
Remco Evenepoel, 22, won the race.
It was a first Monument - one of the five biggest one-day races - win for the Belgian, a Quick-Step team-mate of Alaphilippe, who attacked 30km from the line and rode solo to take a spectacular victory.
"It's crazy," said Evenepoel. "My first ever Liege and I win it."
Nearly a minute behind, Quinten Hermans outsprinted fellow cyclo-cross racer Wout van Aert to take second in an all-Belgian podium.
Britain's Tom Pidcock, Ineos' Olympic mountain bike champion, was one of the first to fall and about 30 riders crashed in total on a fast, downhill section of the course in the Ardennes forest in Belgium.
Double world champion Alaphilippe has now fallen three times in races this season, following crashes at Strade Bianche in early March and La Fleche Brabanconne 10 days ago.
Team-mate Ilan Van Wilder also broke his jaw in the incident on Sunday.
