Evenepoel was part of the leading group when, with around 50km to go, he hit the barriers on the right side of the road during a descent.
- Il Lombardia - LIVE updates from Bergamo to Como
- Lennard Kamna storms to Stage 4 victory at Criterium du Dauphine
He flipped over the barriers and into a segment of forest.
Medical staff and ambulances were quickly on the scene and the rising star was reportedly conscious when they arrived.
TV footage showed him in a neck brace before he was taken to hospital.
His team Deceuninck-Quickstep posted a message saying they will provide updates when they can and asked for fans not to speculate on his condition.
"Thanks to everyone who sent their thoughts and best wishes to @EvenepoelRemco at #ILombardia, it means a lot to us! At the same time, we kindly ask you not to speculate about his condition, when we’ll have an update, we will share it with you."
Later on Saturday they tweeted another update saying that Evenepoel is at Como hospital where he is concisions and his condition is being assessed.
A more concrete update from the team came in the evening, as they confirmed that Evenepoel had suffered a fractured pelvis and a right lung contusion.
"The 20-year-old Belgian suffered multiple injuries during Saturday’s Il Lombardia." The statement read..
"One of the favourites at the start of the season’s second Monument, Remco Evenepoel crashed inside the last 50 kilometers, on the descent of Sormano, hitting a bridge wall and going over it into a ravine. Placed into a precautionary neck brace and taken to the Como hospital by ambulance, Remco was conscious at all times as he underwent a series of examinations to reveal the extent of his injury.
"Unfortunately, the X-rays showed a fractured pelvis and a right lung contusion, which will keep Evenepoel – a winner of four stage races this season – on the sidelines for the upcoming period. Our rider will remain in the hospital overnight under observation, before flying on Sunday to Belgium."
"May the bad luck stop!"
Deceuninck-QuickStep directeur sportif Davide Bramati spoke to Sporza about the incident.
"We didn’t see the fall ourselves, we only heard about it on the race radio. When we saw tha Remco's data had stopped, we already thought it was him. When we got there, all I saw was his bike. A bike without a rider, that's not a pleasant sight," he said.
"I walked straight down and quickly saw that he was okay. He could talk and said he had pain in his right side. He's in the hospital now, with the doctor, Patrick [Lefevere] and his family. I think he'll be all right. But may the bad luck stop for us now,” Bramati said, referring to Fabio Jakobsen's recent crash at the Tour de Pologne.