England head coach Stuart Lancaster will take no risks over the rehabilitation of Mike Brown, who appears increasingly unlikely to play in the crunch RBS 6 Nations clash against Ireland on Sunday.
On Friday Brown felt a recurrence of the concussion symptoms that he suffered after being knocked unconscious in the 47-17 victory over Italy on 14 February.
Having completed a weekend of rest, the full-back has restarted the return-to-play protocols, but the earliest he will be able to resume contact training is Friday, when Lancaster announces his starting XV for the Aviva Stadium showdown between the Championship's only remaining unbeaten teams.
It leaves Brown with a race against time to prove his fitness, knowing any further flare-up in symptoms will force him to start the protocols again and rule him out of contention.
"It's very early for me to pencil Mike in for a game at the moment," Lancaster said at the squad's Pennyhill Park training base.
"We don't know how it is going to play out because he's done stage one of the process on Monday and there could be a reaction. That will have an implication, big time.
"A lot will depend on how the next 24 hours go but a decision will have to be made within the next two days I'd say because the team need a chance to prepare with all the pieces in place.
"It's very early for me to pencil Mike in for a game at the moment" - Stuart Lancaster
"If he ticks the boxes successfully on a daily basis then there'll also be a conversation to be had with him about how he's feeling mentally in terms of the lack of training he's been able to do."
But Lancaster insists he will not rush Brown back into action, despite his first-choice full-back impressing in the opening two rounds of the Championship.
"I've got to be 100% certain that he's right with the type of knock it was and also to go in the size of game that this is," Lancaster said.
"On Mike's behalf I want to be 100% certain he's right. I'm not being cautious, we're just following the protocols to the letter. His welfare is our primary concern and I'm simply guided by what the doctor tells me."
Alex Goode, who won the last of his 16 caps as a replacement against Wales 12 months ago, is on standby to start at the Aviva Stadium with Chris Pennell also in the squad as specialist cover in the position.
Lancaster has discounted the possibility of picking Anthony Watson at full-back and placing Jack Nowell on the wing, although he revealed that Nowell's battle with Jonny May for the number 11 jersey is "very tight".
"We absolutely trust Alex. He's played lots of big games for us in that position," Lancaster said.
"He's lost out in the short term because Mike has played exceptionally well, but Alex has played consistently well for England.
"He's been unlucky in the sense that the standout player in the Six Nations last year was Mike Brown in his position. He's had to wait patiently for his chance again.
"He's a different type of player to Mike in that he's equally good under the high ball, he's very good as a positional full-back and can play fly-half, so he offers that slightly different kicking option, but also the opportunity to be a first receiver/playmaker."
