Maro Itoje has been ruled out of England's Six Nations games against France and Wales with a knee ligament injury.
Itoje suffered the injury during Saturday's 32-20 victory over defending champions Ireland, with referee Jerome Garces initially fearing he had broken his leg.
The 24-year-old forward underwent a scan on Monday, which ruled him out of Sunday's game with France at Twickenham and the trip to Cardiff on February 23.
Itoje will remain at the squad's Surrey training base to continue his rehabilitation and the RFU expects him to be available towards the end of the tournament. England play Italy and Scotland on March 9 and 16 respectively.
"Maro had the scan and saw a specialist yesterday (Monday). It's ligament damage to his knee," England forwards coach Steve Borthwick said.
"We anticipate him being able to play at the latter part of the tournament. Hopefully he'll heal fast and we'll have him back ASAP.
"Which game that is for depends on how his rehab goes over the next couple of weeks, but we'd expect him to feature later on."
Itoje has been replaced by Saracens team-mate Nick Isiekwe in England's 35-man squad for the France game with either Courtney Lawes or Joe Launchbury expected to partner George Kruis in the second row against Les Bleus.
Lawes made a forceful second-half appearance in Dublin but while Launchbury was excluded from the matchday 23, England may want his bulk on from the start.
Meanwhile, Dylan Hartley will join up with England camp this week as he continues his rehab from knee injury, but the hooker will not be involved against France.
England squad
Forwards: Jack Clifford, Dan Cole, Luke Cowan-Dickie, Tom Curry, Ben Earl, Ellis Genge, Jamie George, Nathan Hughes, Nick Isiekwe, George Kruis, Joe Launchbury, Courtney Lawes, Ben Moon, Brad Shields, Kyle Sinckler, Jack Singleton, Billy Vunipola, Mako Vunipola, Harry Williams, Mark Wilson.
Backs: Chris Ashton, Mike Brown, Joe Cokanasiga, Elliot Daly, Ollie Devoto, Owen Farrell (c), George Ford, Jonny May, Jack Nowell, Dan Robson, Henry Slade, Ben Te'o, Ollie Thorley, Manu Tuilagi, Ben Youngs.