England lock Geoff Parling has been ruled out of Saturday's opening November international against Australia at Twickenham due to concussion.
Parling was concussed during a training session at England's base in Bagshot, south-west of London, on Tuesday and will now follow the standard procedure for players who've suffered head injuries.
"Geoff Parling sustained a mild concussion during a contact session this afternoon and will undergo the recommended graduated return to play protocol," said England coach Stuart Lancaster in a Rugby Football Union statement on Tuesday.
"As such, he will be unavailable for Saturday's game but will stay in camp with a view to returning to training next week."
The absence of line-out specialist Parling, a member of the British and Irish Lions side that defeated Australia 2-1 in a three-match series earlier this year, appears to have made what was a problem selection a more straightforward task for Lancaster.
Parling had been competing for a starting spot with Joe Launchbury and Courtney Lawes. It now seems as if the latter two will be England's second row when Lancaster unveils his side to play 2015 World Cup pool opponents Australia on Thursday.
Dave Attwood, only back from a rib injury last week, is in line to provide bench cover.
Lancaster released 11 players back to their clubs for Premiership duty this weekend, leaving him with a squad of 23 where all that is now to be decided is the division between the starting XV and eight replacements.
Midfield, with both Manu Tuilagi and Brad Barritt - England's centres in their stunning upset win over world champions New Zealand during the 2012 end of year series - both injured, again looked like being a problem area.
But Lancaster appears to have opted for a pairing of Gloucester playmaker Billy Twelvetrees and rugby league convert Joel Tomkins after Luther Burrell, Henry Trinder and Kyle Eastmond were all sent back to their clubs.
Another Premiership bound player is Gloucester fly-half Freddie Burns, with Lancaster putting his faith in the tried and tested No 10 duo of Owen Farrell and Toby Flood.
Meanwhile Lancaster must decide between Ben Youngs and Lee Dickson at scrum-half, with Danny Care heading back to Harlequins.
"It's never easy deciding who to select and that's what you want - players pushing hard for places across the board," Lancaster said.
"We have had two good days of training here at Pennyhill Park and, added to last week in Leeds, we are in good form and looking forward to the game."