Scotland flanker Hamish Watson has been ruled out of the rest of the Rugby World Cup because of a knee injury suffered in Sunday's defeat by Ireland.
The 27-year-old back-row will be replaced in the squad by Edinburgh club-mate Magnus Bradbury.
Watson fell awkwardly after being cleared from a ruck and left the field on a stretcher just before half-time.
Scotland head coach Gregor Townsend said: "We feel for Hamish, who had worked so hard for this opportunity."
A scan after the game confirmed the damage to the knee was significant enough to mean Watson will play no further part in the tournament.
"While there is a high probability of picking up injuries during a World Cup, it's always disappointing to see someone having to head home," Townsend added.
Bradbury is already in Japan, having travelled as precautionary injury cover for Jamie Ritchie, who missed the opening game with a facial injury suffered in the final warm-up match against Georgia.
Ritchie is expected to be fit to face Samoa in Scotland's second group game next Monday (11:15 BST).
BBC Scotland chief sports writer Tom English
Having missed the entire Six Nations this year with a broken hand, Watson is now out of the World Cup after less than 40 minutes of action. To call it a devastation is to put it mildly.
Watson's appearance on the international scene came a fraction too late for inclusion in Vern Cotter's squad four years ago. Now he's gone early in Japan.
The brutal nature of rugby means there's no guarantee that Watson will still be in his pomp in four years' time in France. He'll be 31 going on 32 then with a lot more miles on the clock.
It's a cruel injustice on a terrific player, the kind of uncompromising character that Townsend desperately needs in the wake of such an awful night against Ireland in Yokohama.
The flanker brings aggression with ball in hand and scavenging qualities at the breakdown. Scotland are crying out for more of him, not less of him. He's one of the best open-side flankers around and was going to be critical in the difficult weeks ahead.
Finn Russell is the most irreplaceable player in this Scotland squad, but Watson is probably second on that list. Townsend doesn't have anyone else like him. For player and coach, this is a calamity.