Cole Palmer was withdrawn from the Chelsea XI to face West Ham at the London Stadium just minutes before kick-off after being seen trudging off early from the team's warm-up.
The Blues talisman, 23, did not return from the tunnel until shortly before kick-off, where he sat among the travelling delegation with his hood up.
Sky Sports confirmed that Palmer was out of the game before later confirming the England star "felt something" during the warm-up and was removed from the team as a precaution.
Willian Estevao was confirmed as Palmer's replacement just before the game kicked off. Before Palmer's withdrawal was confirmed, Jamie Redknapp said on Sky Sports: "We don't know the details. I was watching him in the warm-up and he wasn't striking the ball with his normal ferocious style.
"Something isn't quite right. We will see. If he isn't playing, Graham Potter will be the happiest man in this stadium."
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He later added: "For neutrals it's such a disappointment. But for West Ham it is great news for the manager. He's their best player, talisman, the man who makes things happen. "It changes things - but Estevao is a tremendous talent as well."
Palmer has established himself as one of Chelsea and the Premier League's most influential players since joining from Manchester City in 2023, so his absence was a major blow for Enzo Maresca's side.
And things went from bad to worse just six minutes as Lucas Paqueta fired a thunderbolt past Robert Sanchez in the away goal to hand West Ham an early lead.
Focus will now turn to how much game time - if any - Palmer will miss following the decision to take no chances with his fitness against the Hammers.
Before the match, Maresca said improving Chelsea’s conversion rate could determine whether they build upon last season’s fourth-placed Premier League finish.

The Blues have spent around £160million on attacking players, all of whom played some part in last Sunday’s lacklustre 0-0 draw with Crystal Palace at Stamford Bridge.
Despite a new look forward line, the difficulty the home side had in breaking down Palace bore hallmarks familiar to supporters who frequently watched their team labour to create and convert opportunities last season.
Chelsea ranked second in last season’s Premier League in terms of big chances missed and placed 11th in percentage of shots scored.
Their tally of 64 league goals was beaten by Brighton and Brentford and matched by 17th-place Tottenham, whilst champions Liverpool outscored Maresca’s team by 22.
“I don’t know if we (now) have more capacity (to score), but for sure if we’ve made changes it’s because we think that with changes we could improve out attacking phase,” said Maresca.
“I was quite happy last year in terms goals we scored but with the amount of chances we created, I think we were one of the best in the league.”
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