Mako Vunipola must undergo hamstring surgery as he races against time to prove his fitness for England's pursuit of the World Cup.
Saracens have revealed that Vunipola will need "a minimum of three months" of rehabilitation once the operation is completed, casting doubt over his involvement in this autumn's global showpiece.
England head coach Eddie Jones will give his most effective player and the game's standout loosehead prop every chance to secure a place in his squad for Japan 2019, but any setbacks in recovery could be damaging.
Vunipola lasted half an hour of Saturday's Champions Cup final victory over Leinster and while Saracens were initially more upbeat over the severity of the torn hamstring, a visit to a specialist revealed the true extent of the damage.
"Mako's hamstring injury requires surgery and will take a minimum of three months to recover," a joint statement issued by England and Saracens read.
England fly to Japan on September 8 and play their opening match against Tonga two weeks later.
Even if Vunipola's recovery follows the best case scenario he will be forced to miss two of the four warm-up matches, home and away against Wales, leaving the August 24 and September 6 appointments against Ireland and Italy as possible opportunities to gain match fitness.
Jones is due to name his World Cup squad in late August but can make changes until the official deadline of September 8 - the day they depart for their pre-tournament training camp in Miyazaki.
England's head coach can take some comfort in knowing that the tougher pool games - against Argentina and France - arrive at the end of the group campaign, offering Vunipola time to bed in.
The 28-year-old Lion missed a large chunk of the Six Nations earlier this year with an ankle problem and then suffered another injury to the same joint that resulted in an aborted comeback.
The showdown with Leinster in Newcastle was only his second match since the Championship rout of France in mid-February and will now sit out the climax to Saracens' quest for the double through the successful defence of their Gallagher Premiership crown.
Vunipola is among England's most influential figures due to his exceptional work-rate, power in the tackle, carrying threat, leadership and handling skills.
Alongside Owen Farrell, Maro Itoje and his younger brother Billy Vunipola, he is a world-class player that Jones dare not lose.
Ellis Genge, Ben Moon and Alec Hepburn have deputised for the New Zealand-born front row of Tongan heritage, but all three are Test rookies and Jones might now consider an attempt to persuade veteran Joe Marler to rethink international retirement.