The 29-year-old broke out a shoulder blade and two ribs on Sunday at the Mass Carbolage 62 kilometers.
In addition, he moved into a lung injury.
That announced his team Quick Step in the evening.
His condition is stable.
Alaphilipe stay for the time being for the observation in the hospital.
How long he fails was not communicated.
In the accident, Alaphilippes Belgian teammate Ilan Van Wilder was involved.
He broke his pine and is also treated in a clinic as well.
The Victory in the classic was secured by the Belgian Jungstar Remco Evenepoel, who also drives for Quick Step.