MANCHESTER (REUTERS) - Manchester City's French international left-back Benjamin Mendy is facing a lengthy absence out of action with the club suspecting his knee injury is worse than at first believed.

"After tomorrow, he is travelling to Barcelona to see what he has," said City manager Pep Guardiola.

"The first impression is that it will be a little bit long, unfortunately longer than we expect," he added, declining to give details of the injury.

Premier League leaders City have used Spanish doctor Ramon Cugat to help with some injured players since Guardiola, the former Barcelona manager, joined the club last season.

Mendy, who has made an impressive start to his career at City following his move from Monaco, for a reported £52 million pounds (S$94 million) fee, missed Tuesday's 2-0 win over Shakhtar Donetsk in the Champions League.

"It is very frustrating for him first as a person. I feel sad for him, like I felt for Ilkay Gundogan," said Guardiola, referring to the German midfielder who missed most of last season with injury. "

For the way we want to play, we will lose a lot, he is a unique player.

"We have alternatives in our squad, Danilo, Fabian Delph, Fernandinho, even (Ukrainian midfielder Oleksandr) Zinchenko," added the Spaniard.