It is a huge blow piled on top of the trauma of Yokohama, in which the Scots were dismantled 27-3 by the world's top-ranked team in their opening pool game.
Watson travelled with the squad on crutches as they took the bullet train to Kobe, venue of their next match against Samoa next Monday, but will soon return to Scotland and is replaced in the squad by Magnus Bradbury, his fellow Edinburgh back-rower, who has been with the squad as standby, initially for Jamie Ritchie, who is now back in full training.
Scotland coach Gregor Townsend said: “While there is a high probability of picking up injuries during a Rugby World Cup, it is always disappointing to see someone having to head home.
“We feel for Hamish, who had worked so hard for this opportunity, and wish him well in his recovery."
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