Goalkeeper Emiliano Martinez was taken to hospital for "precautionary tests" after suffering what boss Xabi Alonso said was a concussion during Chelsea's win at Everton on Saturday.
Martinez had earlier played his part in the Blues securing a first Premier League clean sheet in nine months, a run stretching back 23 matches.
But the incident overshadowed an important 2-0 victory for Alonso's side at Hill Dickinson Stadium, secured by two goals from Pedro Neto.
Martinez collided with Everton defender Jarrad Branthwaite in the 68th minute while attempting to claim a cross from Kiernan Dewsbury-Hall's free-kick.
There was an eight-minute stoppage before the Argentina international was replaced by Mike Penders, who made his Premier League debut for Chelsea, and saw out the remainder of the match.
Asked about the incident after the match, Alonso said: "It was a concussion, but beyond that I do not know. I have only seen one clip of the incident, so we need to wait.
"What we know is that he left the stadium conscious and is now in hospital undergoing precautionary tests.
"We need to wait. Hopefully it will not be too serious, but at the moment we just have to wait."
There are now doubts over whether Martinez will be fit for Chelsea's next three matches - they face Tottenham and Manchester United in the Premier League, with a Carabao Cup tie at Liverpool in between.
Chelsea recorded their first Premier League clean sheet for nine months, since a 2-0 win against Brentford in January.
Although Alonso said he was "not obsessed" with keeping a clean sheet, his side, who had conceded a league-high 13 goals before kick-off, were keen to address that record after a much-improved 5-1 win over Bournemouth following the international break.
There had also been discussion about Chelsea being a "soft" team, particularly away from home in the north of England, having lost 19 of their previous 41 matches north of Luton over the past four years.
Last season, Chelsea lost 3-0 at Everton during Liam Rosenior's five-match winless run, a sequence that ultimately cost him his job. The Blues had also won just one of their past nine matches at the Toffees.
This time, however, Everton managed just six shots, their fewest in a Premier League game since recording five against Arsenal last December.
Chelsea also conceded only one clear-cut opportunity and allowed their lowest expected goals against (xG) figure against top-flight opposition this season at 0.45.
They started with a back three of Josh Acheampong, Maxence Lacroix and Levi Colwill, while goalscorer Neto and Jorrel Hato, who provided the assist, operated as wing-backs.
The midfield pairing of Moises Caicedo, returning from international duty, and new signing Jordan Henderson also brought much-needed solidity and experience.
It was £117m club-record signing Morgan Rogers, however, who set the tone for a combative display, squaring up to Everton defender James Tarkowski during an eventful first half.
"We needed to show that we can win in this way," Alonso added. "We looked solid, we played solidly, we were strong and we stayed in control.
"In the first half there were difficult periods with throw-ins, corners and pressure from Everton. You have to overcome those moments.
"In the Premier League it is very hard to have complete control in an away game, but you cannot lose your shape or your conviction."
With difficult matches to come, there will be questions over whether Penders, who became Chelsea's youngest goalkeeper to play in the Premier League at the age of 21, is ready for the demanding run of fixtures ahead.
That concern would be heightened if tests confirm Martinez suffered a concussion, with Premier League protocols typically requiring a minimum 14-day return-to-play process.
Chelsea signed Penders from Genk for £17m in 2024 before loaning him back to the Belgian club. He then spent last season on loan at sister club Strasbourg, where he impressed in both the Conference League and Ligue 1.
Genk have developed a reputation for producing elite goalkeepers, and Penders is frequently compared to Thibaut Courtois by those who know him best.
Like the Real Madrid goalkeeper, Penders stands at 6ft 7in and has recently served as his understudy in the Belgium national team set-up.
He has worked with the same legendary youth goalkeeping coaches, Guy Martens and Gilbert Roex, who also helped develop Koen Casteels, Maarten Vandevoordt, now at RB Leipzig, Lucca Brughmans, who will join Liverpool next summer from Genk, and Nordin Jackers of Club Brugge.
They specialise in developing goalkeepers who are good with their feet, including Penders, who recorded the highest average position while playing out from the back under Rosenior at the Ligue 1 club early last season.
Roex, who has known Penders since his academy days, believes he is capable of matching Courtois' level of success at Chelsea, Atletico Madrid and Real Madrid.
He said: "We have produced a lot of great goalkeepers, but Mike is the one with the potential to reach the same level as Thibaut. His sense of calm is gold in goalkeeping."