England full-back Mike Brown appears unlikely to face Ireland in the Six Nations on Sunday following a setback in his recovery from concussion.
Brown, 29, was stretchered off in the first half of England's 47-17 win over Italy at Twickenham on 14 February.
He experienced concussion symptoms last Friday, meaning he had to re-start his recovery and will not resume full training until the end of the week.
Worcester's Chris Pennell has been called up to the squad as cover.
"The decision will obviously centre around Mike's health and wellbeing first and foremost," England head coach Stuart Lancaster told BBC Radio 5 live.
"If he is good to go then I have a selection decision to make on the basis of [someone else's] opportunity to train and play fully in the squad during the first part of the week.
|Headlines: Six stages to the GRTP (Graduated Return to Play) as per World Rugby guidance|
|1. Minimum rest period||4. Non-contact training drills|
|2. Light aerobic exercise||5. Full practice contact|
|3. Sport-specific exercise||6. Return to Play|
"The first decision will be whether he is medically fit and able to play in the game, and we will make that decision via the protocol.
"We do need to get some consistency and continuity in the team during the course of Tuesday and Wednesday, but we will wait and see."
Alex Goode of Saracens is in pole position to start at full-back if Brown, who plays his club rugby for Harlequins, is left out.
"He would be the obvious one having played for us recently, but Chris Pennell comes in as well, and he was excellent for the Saxons [against the Ireland Wolfhounds last month]," Lancaster added.
Meanwhile Jack Nowell is pushing hard for Jonny May's place on the wing, while Tom Wood, Geoff Parling and Courtney Lawes are all fit to train this week after injuries.
Of the three, flanker Wood is the one most likely to force his way into the matchday squad after returning from an ankle problem as the two unbeaten sides in the Six Nations prepare to meet in Dublin.