It continues a terrible week for the England head coach, with captain Owen Farrell and Billy Vunipola handed bans after their red cards against Wales and Ireland respectively to leave his plans for the start of the tournament in tatters.
Both players will miss England’s World Cup opener against Argentina in Marseille on 9 September, while Farrell will also be suspended for England’s second pool game against Japan in Nice eight days later.
Borthwick is, however, optimistic that both Elliot Daly (knee) and Henry Arundell (back) will recover from injury in time for the start of the tournament.
Watson’s back three colleagues will miss this weekend’s final warm-up fixture against Fiji at Twickenham, with Jonny May called in to the squad having missed out on selection in the initial World Cup 33.
England will not confirm Watson’s replacement until after the game, while there will also be doubts over the readiness of Tom Curry ahead of the tournament.
The back rower is yet to return to fitness after twisting an ankle at the start of August and has not featured in any of the four warm-up games.
Watson’s latest issue is another major blow for a player who missed nearly a year of action after rupturing his anterior cruciate ligament (ACL) in the autumn of 2021.
The 29-year-old is also currently yet to sign a deal with a club for next season, with the Rugby Football Union (RFU) understood to have been paying Watson’s wages while in camp ahead of the World Cup.
Watson signed a one-year deal with Leicester Tigers before the start of last season and this latest injury could leave his playing future in limbo.
May, who started opposite Watson on the wing in the 2019 World Cup final, is the likeliest beneficiary, though Joe Cokanasiga and Cadan Murley were also part of a wider squad for much of the summer.
Centre Guy Porter, meanwhile, has also been training with England as injury cover this week.