Mason Mount's Champions League debut for Chelsea lasted just nine minutes before he was taken off with an ankle injury.
The 20-year-old England midfielder suffered the knock after an ugly challenge from Valencia's Francis Coquelin in the Blues' 1-0 defeat at Stamford Bridge.
Mount was on the end of a high tackle by the former Arsenal player, who received a yellow card from referee Cuneyt Cakir.
Mount, who has established himself in the first team under Frank Lampard since a loan spell with Derby last season, will be assessed over the next couple of days to determine the extent of the injury.
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Lampard said: "He's got an ankle injury which we don't know how bad it is.
"We will have to assess it over the next 24-48 hours to see the scale of the injury, to see how the injury is.
"It's too soon to say, but it's an ankle injury and obviously it's a shame because he was playing so well and started the game well and we had to make a change early."
Mount got extensive treatment on the pitch and received a standing ovation as he limped off to be replaced by Spanish winger Pedro at Stamford Bridge.
It is a concern for Lampard ahead of Sunday's visit by Liverpool in the Premier League, live on Sky Sports.