Fly-half Marcus Smith has been ruled out of England's Six Nations opener against Italy in Rome on Saturday with a calf injury.
Smith, who had been expected to start against the Azzurri, was injured in training on Monday.
His absence means Northampton's in-form Fin Smith, 21, is set to make his international debut in the Stadio Olimpico.
England will confirm their starting XV and replacements on Thursday evening.
"[Marcus] won't make this weekend, sadly," said England assistant coach Kevin Sinfield.
"We got the scan report through but we are waiting to understand exactly what that means for us moving forward. He's not fit for this weekend."
As well as Fin Smith, England can also call on the vastly experienced George Ford, but Marcus Smith's absence is another setback for head coach Steve Borthwick.
England are already without the whole of the midfield axis that helped them to the World Cup semi-finals and an eventual third-placed finish, with Owen Farrell and Joe Marchant unavailable, and Ollie Lawrence and Manu Tuilagi injured.
The retired Courtney Lawes has also left a void in the back row, as has the injured Tom Curry, while Jonny May's retirement has opened a spot on the wing.
It means Borthwick is set to name a handful of new caps in his matchday squad, with Fin Smith set for a place in the 23 and his Northampton team-mate Fraser Dingwall likely to start at inside centre.
However, captain Jamie George says he has total confidence in whoever is selected, and is buoyed by the performances of the English club sides this season both domestically and in the Investec Champions Cup.
"I've been so impressed with what I've seen from the boys," George told the Rugby Union Weekly podcast.
"But I kind of knew that coming into camp because of the way the English teams have performed and the competition in the Premiership."
"I'm sure the coaches have got some interesting selection headaches, but some players are playing really well for their club," added scrum-half Danny Care. "I'm excited to see what can happen.
"Whoever gets picked, this is a really exciting time to be an England fan and an England player."
England Six Nations XV
Pick your starting XV for England's Six Nations opener against Italy