England suffered a blow ahead of their Six Nations clash with Wales on Saturday after Courtney Lawes was ruled out with a potentially tournament-ending injury.
Lawes was set to continue at blindside flanker in Cardiff but suffered a pectoral, or chest muscle, injury during training on Wednesday.
"The first part of training yesterday (Wednesday) he slipped over and felt something in his pec," England coach Eddie Jones said after announcing his team on Thursday.
"We weren't sure of the significance of the injury so he was scanned this morning at the first available time and unfortunately he's got a little bit of damage there and needs further diagnosis and checking.
"That will be done over the next few days and then we will have a clearer picture of his further participation in the tournament."
The veteran Australian coach added: "He's done some damage to the pec muscle. Sometimes those can be rather serious and sometimes they can be not so serious -- we'll just have to wait and see until we get further information."
Northampton back-row Lawes' place in the team has been taken by Mark Wilson, who returns after being dropped for the defending champions' second-round win over Italy.
"Well, six (blindside flanker), when we play Courtney there, it's more of a jumping, running role," said Jones.
"But normally if it's not Courtney we have more of a work-rate six, which is Mark Wilson. So, a lot of a good defence, a lot of good clean-outs. He will be a bit of a 'glue' player for the team."
The only other change to England's XV is the recall of hooker Jamie George in place of Luke Cowan-Dickie.
But uncapped Leicester teenage back-row George Martin could make his England debut off the bench.
"He’s very much an old fashioned six -- good defensive ability, carries the ball hard and is also a line-out option and with time he may mature to be able to play lock as well," said Jones.
- Triple Crown -
Victory for Wales, who are due to name their team later on Thursday, would seal the Triple Crown and leave them on course for the Grand Slam.
Jones said England, who suffered a shock loss at home to Scotland in their tournament opener, and Wales were under similar levels of pressure for the "make-or-break" game.
"They (England-Wales matches) are tight contests that go down to the wire, down to the last moment. We won one there in 2017 basically 'on the bell'.
"And we had to win 'on the bell' at Twickenham last year so we're expecting a similar sort of game.
"It's the sort of game you've got to win not once, but maybe two or three times. Wales will keep coming and keep coming. It will be a great contest."
Elliot Daly; Anthony Watson, Henry Slade, Owen Farrell (capt), Jonny May; George Ford, Ben Youngs; Billy Vunipola, Tom Curry, Mark Wilson; Jonny Hill, Maro Itoje; Kyle Sinckler, Jamie George, Mako Vunipola
Replacements: Luke Cowan-Dickie, Ellis Genge, Will Stuart, Charlie Ewels, George Martin, Ben Earl, Dan Robson, Max Malins
Coach: Eddie Jones (AUS)