Injured England full-back Mike Brown will return home from the tour of South Africa later this week.
The Harlequins man, 26, broke his right thumb during Saturday's 22-17 first Test defeat in Durban.
Bath's Nick Abendanon, who was already on holiday in South Africa, has been called up as a replacement and will join the squad on Monday.
England have also lost centre Brad Barritt for next Saturday's second Test after surgery on a lacerated eyeball.
Head coach Stuart Lancaster said of Brown: "We're still waiting for specialist opinion as to whether he needs surgery, but the reality is that he's out for six to eight weeks.
"It's very disappointing for him. He's worked hard for his opportunity and to get his place in the starting XV.
"I've told him to remember the quality of the season he's had and to keep in mind that he's got a lot to be proud of this season."
But Lancaster is more optimistic about Saracens centre Barritt after the RFU confirmed on Twitter the 25-year-old will be ready for the third Test in Port Elizabeth on 23 June.
The Saracens man was forced off after a collision in the 54th minute of England's 22-17 defeat at Kings Park.
"He's positive and upbeat," Lancaster said. "It could have been worse news because it looked bad at the time with his eye closing up straight away."
Barritt's absence at inside centre will force a reshuffle of England's backline ahead of the second Test.
Lancaster could switch Manu Tuilagi from outside centre to take Barritt's place, with Jonathan Joseph - who came on for his Test debut for the final few minutes in Durban - an adventurous option at 13.
Alternatively fly-half Owen Farrell could switch to 12, where he ended up on Saturday and created the last-minute try for Ben Foden, with Toby Flood - who came on at 10 when Barritt departed - starting in the pivotal role.
The England side for their midweek tour match against the Southern Barbarians in Kimberley on Wednesday is due to be announced on Monday morning.