Ireland centre Jared Payne has until Thursday to prove his fitness for their Six Nations clash with England after missing training on Tuesday.
Ulster's Payne suffered a hamstring injury in Ireland's 10-9 defeat to France on February 13 and his participation in Saturday's match at Twickenham appears in significant doubt.
"Everyone trained fully today apart from Jared Payne," forwards coach Simon Easterby said.
"He trained probably 85 to 90 per cent yesterday [Monday] and ran well, but today he's going to do some work with the physios and continue that and we'll give him until Thursday to prove his fitness."
Keith Earls, Stuart McCloskey and Luke Marshall could come in for Payne should he fail to recover in time to face an unbeaten England side as defending champions Ireland seek their first win of this year's competition.
"I think we've got a few combinations we could run with," Easterby added. "Keith Earls has played there and Luke Marshall has done too. I think Robbie Henshaw has shown himself to be fairly adaptable. He's played at 15 this year for Connacht.
"There is a difference defensively between 12 and 13, but we'd feel comfortable that whichever combination we go with that they would have had enough time to gel together.
"We're confident we'll be in a good place whatever the combinations that take the field.
"Jared Payne has been excellent in that role at 13 both in attack, but especially defence, he's been very dominant in that area. He took an early bump against the French and dug in very well to stay on the pitch and he got us out of trouble a few times.
"We're keen to give Jared the opportunity to come through on Thursday, but after that we'll make a decision on what combinations will best fit if Jared doesn't make it."