England manager Roy Hodgson is worried that Kyle Walker will miss the World Cup because of a pelvic injury.
Walker looked set to go to Brazil as Hodgson's first-choice right-back following a series of impressive displays for England earlier this season when Glen Johnson was out through injury.
But the defender suffered his injury during Tottenham's defeat by Benfica seven weeks ago and he has not played since.
"We haven't been told for certain that he will be out for the next six to 10 weeks, but we are concerned that it has been a long time he has been out," said the England manager, who could call up Liverpool's uncapped defender Jon Flanagan if Walker is not fit.
"During that time, he has not been able to do any training with the rest of the team as far as I know. 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."
Walker's Tottenham teammate, Andros Townsend, was ruled out of the World Cup on Wednesday because of ankle ligament damage he suffered against Stoke on Saturday.
"I am very disappointed for Andros," Hodgson said.
"He had a fantastic debut. I have been a bit disappointed he has not had as much playing time over recent months as I was hoping he would get, but to see him ruled out like this with a serious ankle injury is a cruel blow.
"My thoughts are very much with him, but he is young. There will be lots of other opportunities although I am sure that won't make him feel better because his chance to get into the squad has been taken away from him."
So far three players - Townsend, Theo Walcott and Jay Rodriguez - have been ruled out of the tournament through injury.
Doubts persist over others, including Arsenal duo Alex Oxlade-Chamberlain, who is carrying a groin injury, and Jack Wilshere, who broke his foot during England's win over Denmark in March.
Hodgson is confident the pair will feature in Brazil though.
"Jack Wilshere has been training for the Arsenal team for a while," Hodgson said.
"I spoke to (Arsenal manager) 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."
Hodgson is also confident that Everton defender Phil Jagielka, who has played regularly alongside Gary Cahill for England, will recover from the hamstring injury which has kept him out since Feb 22.
"We have had good reports back (about Jagielka) for the last few weeks," the former Fulham manager said.
"He has had a problem, but he has been participating to a decent level in training.
"Everton have been keen not to throw him back in before he recovers fully. We don't have any fears that he won't be fit to come with us. It seems he might be back in (the Everton team) on Saturday."
Hodgson confirmed on Wednesday that he will announce his World Cup squad on May 12 - one day after the end of the Premier League season.
The 66-year-old will make the announcement at the headquarters of England team sponsor Vauxhall in Luton.