Belgian cyclist Wout van Aert suffered a broken collarbone and several broken ribs in a heavy crash at the Dwars door Vlaanderen one-day race.
The 29-year-old will now miss upcoming classics the Tour of Flanders, Paris-Roubaix and Amstel Gold Race.
Team Visma-Lease a Bike added it is “unclear” how long he will be out for.
Van Aert fell in a high-speed crash 67 kilometres from the finish of the race in Belgium, which was won by Visma team-mate Matteo Jorgenson.
A group of 10 riders in total came down, with Jasper Stuyven, Biniam Girmay and Michele Gazzoli also forced to abandon the race because of their injuries.
Van Aert was in visible pain, with his jersey torn, before he was taken to hospital.
He was one of the favourites for Sunday’s Tour of Flanders, while the participation of fellow contenders Stuyven and Girmay is also in doubt.
Paris-Roubaix takes place on 7 April, with the Amstel Gold Race on 14 April.
The versatile Van Aert, who won the 2021 Amstel Gold Race and has nine individual Tour de France stage wins, was targeting maiden victories at Flanders and in Paris-Roubaix this year.
He will hope to be back in time to make his planned debut at the Giro d’Italia, which begins on 4 May and where he is hoping for stage wins, having said he is set to skip this year’s Tour de France.
Dutch great Marianne Vos won the women’s Dwars door Vlaanderen by beating Shirin van Anrooij in a sprint.