The full-back has not turned out for his club since sustaining a pelvic problem on March 13, leading to fears he may not be fit in time to travel with the squad to Brazil next month.
And England manager Roy Hodgson has conceded that Walker's fitness is a worry, with his 23-man squad to be named on May 12.
"We haven't been told for certain that he will be out for the next six to 10 weeks but we are concerned," he said.
"I know he has had another injection. We are waiting to find out whether that helps. He has not been fit for a long time."
Meanwhile, Hodgson was less concerned over Jack Wilshere, Alex Oxlade-Chamberlain and Phil Jagielka as the trio look to recover from their respective injuries.
"Jack Wilshere has been training for the Arsenal team for a while," added Hodgson. "I spoke to Arsene Wenger, who said he was holding him back.
"And from my conversation with Arsene, I don't think there is too much to worry about with Oxlade-Chamberlain either.
"We have had good reports back for the last few weeks. Everton were keen not to throw him (Jagielka) back in before his injury is 100 per cent right. We don't have any fears he won't be fit."
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