Owen Farrell left England with an anxious wait on his fitness just a day out from their Six Nations international with Scotland at Twickenham as he missed Friday's final pre-match training session.
Farrell, the team's first-choice goalkicker, has been central to England's run of 17 successive Test wins and was named Thursday at inside centre for Saturday's clash against Scotland.
But it then emerged he had suffered an unspecified leg injury and the playmaker missed training on Friday.
Farrell was present for kicking practice conducted by England great Jonny Wilkinson on Friday but was only seen going through the motions of technique rather than striking the ball.
Such is his importance that England are prepared to give the 25-year-old Farrell as long as they can to prove his fitness for a Calcutta Cup match where a win would see them equal New Zealand's record of 18 consecutive Test wins by a tier one nation and could, depending on the result of Friday's clash between Wales and Ireland, see them retain the Six Nations title they won last year with a Grand Slam.
"Owen didn't train today and we're monitoring his leg injury," defence coach Paul Gustard told reporters at Twickenham on Friday.
"We have until tomorrow (Saturday) before kick-off to make a decision, which means he has at least another 24 hours of recovery ahead of him. We'll make a call as and when we know more," Gustard added.
- Ford and Daly options -
England have decided against calling up an additional player as cover, relying on the 24-man training squad they had previously retained for the Scotland match.
If Farrell is ruled out, England's most likely course of action is to bring Ben Te'o in at inside centre and hand goal-kicking duties to fly-half George Ford.
Elliot Daly, a noted long-range goal-kicking specialist, is already in the team as a wing.
"We're not bringing anyone in," said Gustard. "We're obviously very happy with Ben in the squad.
"Whatever combination we end up with at 10, 12, 13 (fly-half, inside centre, outside centre), we're happy."
England coach Eddie Jones, in the course of a tetchy press conference on Thursday, tried to downplay Farrell's injury by saying he had run into his dog at the squad's training base in Bagshot, southwest of London.
The Australian then sowed further confusion over Farrell's fitness by declaring that Farrell "could be a doubt" but that he would also be "alright" to face Scotland within a matter of moments.
England remain the lone unbeaten team heading into the penultimate round of the Six Nations but they have yet to hit top form, with none of their three previous matches this Championship producing a half-time lead.
"We've done a lot of good stuff...Everyone is looking forward to playing Scotland," said Gustard.
"Scotland have got some good players, but then every team does. We're trying to focus on our own principles and philosophies."
The Scots have not won at Twickenham since 1983 but remain title contenders after defeating both Ireland and Wales this season.