Chelsea have announced captain Reece James is set for "a few weeks" on the sidelines after suffering a hamstring injury in training.
Mauricio Pochettino confirmed the news at his press conference on Friday but backed his skipper to return stronger.
He said: "It's a hamstring. It's not a few days, a few weeks maybe.
"I think it's a sad moment because he's our captain, he was so excited to be the captain, full of energy. We are going to assess him day by day.
"He is going to come back stronger than he was."
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James has started rehabilitation having undergone assessments on the injury.
The full-back missed a substantial amount of last season with hamstring, knee and thigh problems and this latest setback will be another blow for Blues boss Mauricio Pochettino, who has already lost Christopher Nkunku and Wesley Fofana to long-term injuries.
He was withdrawn in the second half of Chelsea's opening weekend draw with Liverpool and was replaced by 20-year-old Malo Gusto, who made his debut for the club.
Neville: Chelsea have potential | James, Chilwell injuries would destroy plans
Speaking on his podcast after the game, Sky Sports pundit Gary Neville warned about the potential damage an injury to James or fellow full-back Ben Chilwell could do to Chelsea's prospects this season.
"What we've seen is that Chelsea have fantastic wing-backs, but they've both gone off in the game," said Neville. "First game of the season. That will destroy Pochettino's plans.
"If you're going to play wing-backs they have to be outstanding players of high quality, who attack like wingers and defend like full-backs, and they have that. But the big question mark is if those two can stay fit.
"Those wing-backs are crucial and if they lose them the whole thing could fall apart a little."
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What's next for Chelsea?
Chelsea visit West Ham next Sunday August 20 live on Sky Sports; kick off 4.30pm.
Chelsea's next six fixtures...
- Sunday Aug 20: West Ham (A) - Premier League, kick-off 4.30pm, live on Sky Sports
- Friday Aug 25: Luton (A) - Premier League, kick-off 8pm, live on Sky Sports
- Wednesday Aug 30: AFC Wimbledon (H) - Carabao Cup, kick-off 7.45pm
- Saturday Sep 2: Nottingham Forest (H) - Premier League, kick-off 3pm
- Sunday Sep 17: Bournemouth (A) - Premier Leaguem, kick-off 2pm, live on Sky Sports
- Saturday Sep 23: Aston Villa (H) - Premier League, kick-off 3pm
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Keep up to date with all the latest transfer news and rumours in our dedicated Transfer Centre blog on Sky Sports' digital platforms. You can also catch up with the ins, outs and analysis on Sky Sports News.