Mason Mount described Ben Chilwell's hamstring agony as "tough to watch" and hopes his Chelsea and England team-mate can recover in time to feature at the World Cup.
Left-back Chilwell was forced from the field during stoppage time in the Blues' 2-1 win over Dinamo Zagreb in the Champions League on Wednesday.
With the Qatar 2022 finals less than three weeks away, Chilwell faces a race to prove his fitness for England boss Gareth Southgate, deepening what is already becoming a defensive crisis for the Three Lions.
Mount was left fearing the worst for Chilwell, but he remained hopeful it may not be as serious an injury as it looked at first glance.
"We don't know how bad it is, but that was tough to watch," Mount told Chelsea's official website. "We saw his reaction and that never looks good.
"As soon as you feel something and to not even think about the ball, it's tough. We know what's coming up, hopefully it's not too bad and he can be back as soon as possible because we need him as well."
While Chilwell's place on the plane now looks in doubt, one player who has given his tournament prospects a shot in the arm is Raheem Sterling.
The England winger broke his Blues drought with a goal in the midweek win, and Mount was delighted to see Sterling on the scoresheet for the first time since September.
"It's brilliant for Raz," Mount said. "He took his goal very well. He doesn't think about how many games he goes without scoring.
"He's so focused on helping the team and getting in the right positions. It will come for him, we know how good he is."
Chelsea next play Arsenal on Sunday in the Premier League. England will play their opening World Cup game against Iran on November 21.