The incident occurred in the Arenberg forest and checks at Valenciennes hospital revealed a nasty contusion on the Italian's chest along with a cut on the back of his hip.
Video footage (as seen below) captured the moment when Viviani approached a pile-up, caused by a crash up ahead, and was struck by the motorbike from behind. He was able to extricate himself pretty quickly but, having been in one of the early breaks, that marked the end of his race.
"Today I was unlucky but at the same time the most lucky of the world!" wrote Viviani on Twitter, also posting pictures of the crash and his war wounds.
Sky directeur sportif Servais Knaven spoke to Cyclingnews and another journalist outside the team bus and expressed his dismay. "It should not be possible," he said. "If you go in Arenberg there should not be any motorbikes near the riders."