Manchester City fear left-back Benjamin Mendy could face a long lay-off with a knee injury.
The France international was forced off during the first half of Saturday's Premier League victory over Crystal Palace at the Etihad Stadium after being hurt in a tackle on Andros Townsend.
City hoped the problem would not be serious, and there was even talk of a fitness test prior to Tuesday's Champions League clash with Shakhtar Donetsk, but it is worse than initially thought.
The 23-year-old will now travel to Barcelona for a full assessment with knee specialist Dr Ramon Cugat, but manager Pep Guardiola concedes the situation does not look promising.
Speaking after the 2-0 win over Shakhtar, Guardiola said: "The day after tomorrow he is travelling to Barcelona and we are going to see finally what he has.
"Our first impression (is he) will be a little bit long, out of the picture, unfortunately for us.
"I don't know, we will wait until Thursday, but I am pretty sure it will be longer than we would like, unfortunately."
Mendy, who has made a positive impression since his £49.1million summer move from Monaco, attended the Shakhtar game on crutches.
He will become one of a number of City players to have been seen by the world-renowned Dr Cugat, who is a close friend of former Barcelona boss Guardiola, over the past year.
Gabriel Jesus, Vincent Kompany and Kevin De Bruyne are among those to have been treated by him.
On the positive side for City, midfielder Fabian Delph produced an impressive performance as a makeshift left-back in Mendy's absence on Tuesday.
Guardiola said: "I have a theory - midfield players, when they are intelligent, can play everywhere because they understand the global situation.
"He played an amazing game. I am so happy for him. Last season he was injured a lot of the time and when he was fit maybe I didn't count too much on him.
"That was my personal regret. He deserved more minutes."