Mako Vunipola will miss the climax to double-chasing Saracens' pursuit of the Premiership title after being ruled with a "significant" hamstring injury.
Vunipola was forced off after half an hour of last Saturday's Heineken Champions Cup final win over Leinster.
But Saracens director of rugby Mark McCall insists the British & Irish loose-head will be available for England's World Cup campaign.
Fellow prop Titi Lamositele will also miss the Premiership title push.
"Mako has a torn hamstring," said McCall. "We're waiting to see exactly what happens with that, but I very much doubt he'll play again this season. Neither will Titi.
"Mako won't be out of the World Cup, but he'll be out for a while. It's quite a significant injury. It's disappointing to lose two players of that quality. It makes us a little bit vulnerable."
Vunipola missed a large chunk of the Six Nations with an ankle injury and then suffered another injury to the same joint that resulted in an aborted comeback.
His appearance at Newcastle's St James' Park was only his second match since the Six Nations win over France in mid-February.
Both players were due to be rested as part of sweeping changes for the climax to the regular Premiership season at Worcester on Saturday.
They will now miss a home play-off semi-final on 25 May and, if Sarries progress, the final at Twickenham a week later.