World Cup 2015: Ospreys' Eli Walker not fit to face Boks
|Rugby World Cup quarter-final: Wales v South Africa|
|Venue: Twickenham Date: Saturday, 17 October Kick-off: 16:00 BST|
|Coverage: Live on BBC Radio Wales, Radio Cymru and BBC Radio 5 live, plus live text commentary on the BBC Sport website.|
Wales have called Ospreys wing Eli Walker into their World Cup squad after Liam Williams was ruled out.
But Walker's hamstring injury means he cannot face South Africa in Saturday's quarter-final at Twickenham.
Once-capped Walker, 23, can only now play if Wales reach the last four and coach Rob Howley confirmed: "He's not available this weekend."
George North and Alex Cuthbert are set to be Wales' wings, with James Hook or Tyler Morgan at outside centre.
Free for recall
Walker was in Wales' initial 31-man squad, but then omitted because of his injury.
Having been capped by Wales at Under-16, 18 and 20 level, Walker's progress was disrupted by shoulder and hamstring injuries.
The Osprey's debut came in Wales' World Cup warm-up defeat by Ireland in Cardiff in August 2015.
But after making coach Warren Gatland's squad following full-back Leigh Halfpenny's knee injury, a fresh hamstring problem ruled out Walker.
However, his withdrawal from the initial squad more than 48 hours before Pool A began meant Wales were free to recall Walker at any stage.
Gloucester back-rower Ross Moriarty filled Wales' 31-man quota after Walker left.
Howley added: "He's [Walker] just got back from the last injury he had when we released him, and he'll be available if needed for the following week."
Amid Wales' centre injury problems, North was at centre in Wales' 15-6 Pool A decider defeat by Australia.
Former Wales scrum-half and captain Howley says North will return to join Cuthbert out wide to help counter the "aerial threat" posed by Springbok wide-men Bryan Habana and JP Pietersen
"If you look at South African strengths and the way they've played the game when we played against them in the autumn and last summer, it's an aerial battle.
"We've decided probably that both George and Alex will be best on the wings to negate that threat.
"When you look at the experience of Habana and JP Pietersen, it's probably trying to match that experience and strength with what we've got."
Howley said Wales will have either twice-capped 20-year-old Newport Gwent Dragon Morgan or versatile 80-cap Hook as Jamie Roberts' midfield partner against South Africa.
"Tyler had a slight injury last week so we've got the option of either Tyler or James Hook into the centre," said Howley.
Roberts has passed concussion tests since coming off late in the defeat to the Wallabies at Twickenham last weekend.
"Jamie's fine... he's gone through the protocol, he'll be fine and available for selection on Saturday," said Howley.
Wales squad ahead of quarter-finals
Props: Tomas Francis (Exeter Chiefs), Paul James (Ospreys), Aaron Jarvis (Ospreys), Gethin Jenkins (Cardiff Blues), Samson Lee (Scarlets).
Hookers: Scott Baldwin (Ospreys), Ken Owens (Scarlets).
Locks: Jake Ball (Scarlets), Luke Charteris (Racing 92), Dominic Day (Bath), Bradley Davies (Wasps), Alun Wyn Jones (Ospreys).
Back-rows: Taulupe Faletau (Newport Gwent Dragons), James King (Ospreys), Dan Lydiate (Ospreys), Justin Tipuric (Ospreys), Sam Warburton (Cardiff Blues, capt), Ross Moriarty (Gloucester).
Scrum-halves: Gareth Davies (Scarlets), Lloyd Williams (Cardiff Blues), Mike Phillips (Racing 92).
Fly-halves: Dan Biggar (Ospreys), Matthew Morgan (Bristol), Rhys Priestland (Bath).
Centres: Tyler Morgan (Newport Gwent Dragons), Jamie Roberts (Harlequins).
Wings: Alex Cuthbert (Cardiff Blues), George North (Northampton Saints), Eli Walker (Ospreys).
Utility backs: James Hook (Gloucester), Gareth Anscombe (Cardiff Blues).