The Omega Pharma-Quick Step rider crashed early in Sunday's Ronde van Vlaanderen and, having been in some discomfort since, underwent scans.
The results revealed the extent of the damage and he has been ruled out of the defence of his 'Hell of the North' title.
"Tom Boonen was diagnosed with a fracture to the 10th rib of the left hemitorax, in addition to the injuries already known after his unfortunate crash at the Ronde on Sunday,” his team confirmed.
"He felt pain in the area since Tuesday and this morning X-rays at Herentals underlined a fracture.”
Boonen has won the infamous cobble-stone race four times.
OPQS will also be missing Dries Devenyns from the Ardennes Classics after he underwent surgery on a complicated fracture to his arm sustained in the Vuelta Ciclista al Pais Vasco on Monday.
"I would like to thank the hospital and medical staff for their professionalism," Devenyns said. "Unfortunately the fracture was complicated and I have to wear a cast for a few weeks.
"It's really a shame, I was in good condition and ready for the Ardennes Classics. There's nothing I can do now, only recover and try to be ready as soon as possible."
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