Maro Itoje is set to start England's RBS 6 Nations opener against France in the back row, but doubt has resurfaced over George Kruis' availability for the Twickenham showdown.
Itoje will move from lock to fill the void at blindside flanker created by Chris Robshaw's Championship-ending shoulder surgery, making his first start in a position viewed by many as his long-term home.
The dynamic 22-year-old, who missed the autumn internationals with a hand injury, made his debut as a replacement in the number six jersey against Italy last year and also switched to the back row in the final Test of the subsequent summer tour to Australia.
England could be forced to rip up their second row plans due to the knee problem that forced Kruis to miss Tuesday's training session at the squad's Surrey training base.
Kruis is to undergo further assessment on Wednesday as he comes to terms with his latest injury setback, having only been given the all-clear to face France last week after recovering from a fractured cheekbone.
If Kruis is ruled out, Courtney Lawes and Joe Launchbury will reform the second row partnership that started against South Africa and Fiji during the autumn.
Tom Wood is poised to continue at openside in a reprise of the role he performed throughout the autumn in the absence of James Haskell, who is himself on course for an unexpected return to the bench.
Haskell has played less than an hour of rugby since returning from toe surgery on January 8 and has been struggling with residual soreness in his foot, but he was given the all-clear towards the end of last week's training camp in Portugal.
Given his seven-month absence, his likely inclusion on the bench is a gamble but the muscular Wasp has been one of the bulwarks of England's success under Eddie Jones and the head coach values him highly.
England will be monitoring the health of Jonny May closely after he was left bloodied by a clash of heads during Tuesday afternoon's training session.
May was certain to start against France after Anthony Watson was ruled out by a hamstring injury.
England 25-man squad retained in preparation for Saturday's Six Nations match against France:
Forwards: D Cole (Leicester Tigers), E Genge (Leicester Tigers), J George (Saracens), T Harrison (Northampton Saints), D Hartley (Northampton Saints), J Haskell (Wasps), N Hughes (Wasps), M Itoje (Saracens), G Kruis (Saracens), J Launchbury (Wasps), C Lawes (Northampton Saints), J Marler (Harlequins), M Mullan (Wasps), K Sinckler (Harlequins), T Wood (Northampton Saints)
Backs: M Brown (Harlequins), D Care (Harlequins), E Daly (Wasps), O Farrell (Saracens), G Ford (Bath Rugby), J Joseph (Bath Rugby), J May (Gloucester Rugby), J Nowell (Exeter Chiefs), B Te'o (Worcester Warriors), B Youngs (Leicester Tigers).