The Swiss national time trial champion was set to play a key role for 2017 winner Greg Van Avermaet, but crashed out before the cobbles had even begun, going down on a left-hand bend on the approach to the first sector of pavé of Troisvilles, after 90 kilometres. Van Avermaet went on to finish fourth, 1:34 behind winner Peter Sagan.
Küng fell on his face, cutting open his chin. He received six stitches in the race ambulance before being taken to hospital in Roubaix for further checks. It was there that X-rays revealed the fracture in his jawbone.
The 24-year-old returned home to Switerzland on Monday and went to hospital in St Gallen, where it was decided he should undergo surgery.
"After consulting the specialists at Kanton Spital Hospital, the surgical option was deemed the best treatment method to optimize the recovery of his injuries. The surgery was performed last night and went well and Stefan will probably go home today," explained BMC's chief medical officer, Max Testa.
"He will now have a couple of easy weeks before rebuilding his training load. Stefan will be restricted to liquids and soft food for a while to minimize the movement of the jaw but he is able to speak.
"Stefan should be able to train on his home trainer in a few days and from there slowly build up and return to the road. We expect him to return to racing in five to six weeks but we will continue to monitor his recovery and alter his program accordingly."
Küng, who dislocated his shoulder at Tirreno-Adriatico, will miss the Tour de Romandie later this month – one of his 'home' races – but his build-up towards the Tour de France should be unaffected. Possible concussion is one potential hangover from the impact that will need to be monitored.
"I'm feeling good and I'm fortunate that these injuries are treatable and I'll be back on my bike soon," said Küng. "I want to thank Dr. Max Testa and the BMC Racing Team medical team, as well as the specialists here in St. Gallen for taking good care of me. I hope to be back racing as soon as possible."