Chelsea manager Frank Lampard has indicated Mason Mount suffered an ankle injury and will undergo further tests to ascertain the extent.
The young Englishman's first European appearance lasted for only 15 minutes and was forced off after a nasty challenge from former Arsenal midfielder Francis Coquelin.
He initially tried to shake it off, but the pain became unbearable as he was replaced by Spanish winger Pedro, leaving the pitch to a standing ovation from the home crowd.
"He’s got an ankle injury but we don’t know how bad it is. We’ll have to assess it in the next 48 hours to see the scale of the injury," Lampard said after the match.
"It was a shame because he started the game well and it meant we had to make the change early on," he added.
Mount has been one of the club's shining stars in the Premier League this campaign, netting three goals from his first five matches.
His form led to his first national team call-up, and his injury will be a big blow to the club, who play Liverpool this weekend in a Premier League match.
In the event, he misses out in the match against Liverpool, American Christian Pulisic, who hasn't featured as much this campaign might get a chance for some game time.