England full-back Mike Brown's chances of facing Ireland this Sunday appear slim after a setback in his recovery from concussion.
Brown had been progressing well after suffering a major blow in a collision during the victory over Italy on February 14.
However last year's Six Nations Player of the Tournament isn't due to be back in full training until Friday, putting his place in doubt.
"Mike is going through the [return-to-play] process now," Lancaster said.
"He was symptom free and then he did the bike on Friday and there were some symptoms after that, so he has had to step down again.
"He's had a rest over the weekend and has started the return to process again as of today. So today [Monday] has been bike, with tomorrow running and ball skills. Non-contact rugby on Wednesday, non-contact rugby Thursday and if all that is fine he's potentially good to go to train on Friday."
Lancaster added that he wouldn't take any risks over Brown's fitness, both because of the importance of the fixture and the player's welfare.
"I've got to be 100 percent certain that he's right with the type of knock it was and also to go in the size of game that this is," added Lancaster.
"The reality is he won't have trained from the game to potentially this Friday which is 13 days. You have to have the team ready to play. On Mike's behalf I want to be 110 percent certain he's right.
"A lot will depend on how the next 24 hours go but it 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. We could leave it until Friday because we don't name the team until then – that's the very latest – but as a coach going into Tuesday and Wednesday you want to be reasonably certain what the team is going to be.
"It will be a judgement call based on a medical call. I think hypothetically it's going to be very early for me to pencil him in for a game at the moment."
Should Brown not be available, as now seems likely, England will turn to Saracens full-back Alex Goode. Worcester's Chris Pennell has also been called up as cover.
"He's [Goode] 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," added Lancaster.
"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 fullback, he can play fly-half (he did at the weekend when Charlie Hodgson pulled out at the last minute), so he offers that slightly different kicking option, but also the opportunity to be a first receiver/playmaker.
"He's certainly confident and he feels he's ready, should his opportunity come, and having played in big games last year for us in the Six Nations last year I think he'd be ready to take his chance."