Owen Farrell will be given until Friday to prove his fitness for Saracens' Champions Cup quarterfinal against Leinster but Billy Vunipola has been ruled out of the trip to Dublin.
Farrell is struggling with a quad injury he suffered in England's final Six Nations match against Ireland and only returned to light training on Wednesday with Alex Lozowski on standby to deputise.
Vunipola had been pencilled in to make his comeback from the fractured arm sustained in January at the Aviva Stadium on Sunday but remains a couple of weeks away from full fitness.
"We'll make a decision on Owen on Friday. His chances are probably 50-50. Billy won't be available," Saracens director of rugby Mark McCall said.
"He did a little bit today [Wednesday] for the first time and we'll see how he reacts to that.
"Fingers crossed he'll be able to do a bit more on Friday. I think he will be able to kick, but place-kicking is going to be the question mark. He doesn't need to be able to place-kick to play.
'I wouldn't want to play him if he's going to break down and be out for a long period of time so we'll be advised by the medical team as to how big a risk that would be.
"Owen is a great player in good form at the moment. He had a good Six Nations and was probably one of England's best players in the competition.
"He's on top of his game and we're hopeful that he'll play, but if he can't Alex is a great stand-in who played really well over the weekend."
Vunipola had been pencilled in to make his comeback from the fractured arm sustained in January at the Aviva Stadium on Sunday but the England number eight remains a fortnight away from full fitness.
"Billy won't be available. He's as fit as he's ever been but unfortunately his arm is still broken," McCall said.
"We're not going to take a risk with him because he's not quite there. He's not far away, probably a couple of weeks away."
McCall, who expects to have George Kruis available after a back problem, insists Saracens have the depth to topple Stuart Lancaster's side despite an injury list that also includes Michael Rhodes, Will Skelton and Calum Clark.
"Of course we're still good enough to win. Although our form this season has been up and down, there have been five or six performances which were really, really high level and we are going to need one of those this weekend," McCall said.