Britain's Mark Cavendish pulled out of the Abu Dhabi Tour after he was concussed and sustained whiplash in a fall during the race's first stage.
Cavendish's team said the crash happened after "the race director's car slowed suddenly as it led the peloton out of Madinat Zayed".
The 32-year-old went on to start the race, but pulled out after five kilometres.
Team Dimension Data say they are monitoring the Manxman's condition.
"Due to the concussion, we were not willing to risk rider safety and the call was made for Mark to stop the race," said team doctor Adrian Rotunno.
The team initially tweeted that Cavendish had fallen on the same shoulder he broke during last summer's Tour de France.
They later deleted that post and replaced it with an update on the rider's injuries.
Cavendish was forced to withdraw from the 2017 Tour de France when he fell and fractured his right shoulder.
He dislocated the same shoulder when he crashed out of the Tour on the opening stage in 2014.
Welshman Luke Rowe finished in the peloton for Team Sky, in his first race since breaking his leg on his brother's stag party.