Chelsea must wait up to 48 hours to discover the extent of the injury that forced Mason Mount off early in their Champions League defeat by Valencia.
Manager Frank Lampard has confirmed Mount, 20, suffered an ankle injury in a challenge with Francis Coquelin.
The midfielder has started all five of Chelsea's Premier League games and made his England debut earlier this month.
"We have to assess it over the next 24-48 hours to see the scale of the injury," said Lampard.
"We don't know how bad it is. It is an ankle injury and it is a shame. He's been playing so well and he started well tonight."
Mount signed a new five-year contract in July and made his first-team debut for Chelsea on the opening day of the season.
He has gone on to score three goals from five Premier League starts, appearing as a substitute in the Blues' Super Cup defeat by Liverpool before his Champions League debut against Valencia.
Former Derby boss Lampard had Mount on loan at the Championship side last season, with the youngster scoring 11 goals in 44 games in all competitions.