Sven Goran Eriksson admitted that losing Jonathan Woodgate from his Euro 2004 plans because of injury would be a "big blow" for England.
However, he has not yet given up hope that both Woodgate and Gareth Southgate, who is also doubtful to play again before the end of the Barclaycard Premiership season, will recover in time for the tournament.
Eriksson was dealt a third central defensive setback in just a few months when Woodgate pulled a thigh muscle in Newcastle's weekend victory against Chelsea.
The Newcastle United manager, Sir Bobby Robson, said: "It is a three to four-week injury but it could be six. Portugal? I am not saying he is out for that. That's in June and we are still in April."
Eriksson admitted: "If he's not available for Portugal then it would be a big blow for us. He's been playing very well recently. I still hope that he can be available but I'm not sure any more."
Woodgate has played just three times for Eriksson, due initially to his international suspension following his court case and then due to a series of injury problems. However, he was in contention with John Terry to replace Rio Ferdinand, who has been ruled out through his ban, while Gareth Southgate is also facing a race against time to recover from a knee injury.
Eriksson, who has a further worry over Sol Campbell's groin injury, is due to name his provisional 23-man squad for the tournament on 17 May, two days after the end of the Premiership season. However, while Ledley King and Wes Brown are therefore - at least theoretically - on standby, he does not have to reveal his final squad until 3 June, giving him two further weeks in which to assess the players' fitness.
The England coach, who has already lost the goalkeeper Chris Kirkland from his plans, says that he already knows at least 20 of the 23 players in his likely squad.
Asked if the fringe candidates had been given enough international chances to impress him, he replied: "Maybe not, but they should show me during the year that they deserve to have more time in the England team."
Jermain Defoe could still miss out for exactly that reason, despite his superb recent form, with Eriksson set to keep faith with Emile Heskey, as well as Darius Vassell, Michael Owen and Wayne Rooney.
"Heskey has had a very difficult season for Liverpool but he has always done a very good job in the competitive games that he's played for England," the Swede told BBC Radio One's Newsbeat show.
"The fact is that we don't have too many attackers with his physical presence in this country. In some games, I think he should be very useful."
The Swede is hoping for two meetings with France at the finals, believing that Jacques Santini's side, who face England in their first group game, are the "big favourites".
As for his own future, having just signed a two-year contract extension until 2008, Eriksson insists that only a run of poor results would force him out of the post despite his recent talks with Chelsea.
"I think the only thing that would make me quit the job is if people want me out if we're losing too many football games," he said.Reuse content