Brussels - Belgium's Etixx rider Tom Boonen was forced to pull out of the Eneco Tour after a heavy fall on Thursday's fourth stage.
The 35-year-old Belgian was eating when his front wheel hit a pothole in the road some 50km into the 202km stage between Aalter et Leeuw-Saint-Pierre.
"The wheel came off and Tom fell," said Etixx manager Patrick Lefevere.
"As my rider felt groggy, the decision was immediately taken to transport him to hospital for more tests."
Boonen, a sprint specialist and winner of many one-day races, was using the Eneco Tour as warm-up for next month's UCI Road World Championships in Qatar where he had been expected to lead the Belgium team.