Walker, 23, was named in the original 31-man squad but was released due to injury.
Wales, who play South Africa in the quarter-finals on Saturday, have suffered a raft of injuries during the tournament, particularly their backs.
Fullback Leigh Halfpenny and scrumhalf Rhys Webb pulled out before the World Cup started and Wales have since lost winger Hallam Amos and centres Scott Williams and Cory Allen, as well as Liam Williams who plays at fullback or on the wing.
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(Reporting by Ed Osmond; Editing by Amlan Chakraborty)
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