The French defender, who was at the Etihad Stadium for the 2-0 win over Shakhtar Donetsk, will fly to Barcelona later this week for more tests on his knee.

Mendy is the only out-and-out left back at the club and if he is ruled out for a significant amount of time, Guardiola insists he does have options for cover.

"Tomorrow he's travelling to Barcelona and we will see finally what he has," Guardiola told a news conference. "Our first impression is that it will it be a little long, longer than we expected. It is very frustrating for him first as a person.

"It is very frustrating for him first as a person but like happened last year with Ilkay Gundogan, I feel really sad for him. For the way we want to play, we will lose a lot, he is a unique player.

"We will see in January [about buying a replacement]. We thought next summer about looking for another left-back, another replacement. We have alternatives in our squad -- Danilo, [Fabian] Delph, Fernandinho, even [Oleksandr] Zinchenko."

City did at least round off the day with a victory to start the Champions League with back-to-back wins for the first time in the club's history.

Guardiola had warned before the game that Shakhtar were dangerous opponents and the Ukrainian champions offered plenty of threat before Kevin De Bruyne's first goal of the season broke the deadlock and a late Raheem Sterling goal wrapped up the win.

"The way they play demands a lot of things," the City boss added. "We beat an amazing team.

"If we hadn't played with that intensity, with high pressing, with that team you cannot beat them. In the first half we had problems controlling the counterattack, but in the second half we were really good."

Guardiola called Delph the man of the match for his performance, adding: "[The] guy is so kind and always tries to help. When he plays in the side we have more tempo and time and he did it really well."

But Delph hailed De Bruyne's "moment of magic" in what proved a testing night against the Ukrainian champions.

"It was very difficult. We're not used to teams who come and keep the ball as well as they do," Delph told BT Sport 3. "We had to be organised and patient but in the end we knew we'd get the chances.

"We probably could have stuck a few more away, but when you've got guys like this guy [De Bruyne] who can have that moment of magic you're always confident going into games."

Belgium international De Bruyne played down his own game while praising visitors Shakhtar.

"Well I definitely think it's not my best game," De Bruyne said. "It's just the way it goes today but this team can definitely play some football. They're very skilful guys.

"The first half was very difficult for both sides but the second half we were better, kept the ball better and created more chances. In the end it was a deserved victory and we keep another clean sheet so that was very nice."

City have six points at the top of Group F, ahead of Shakhtar and Napoli, who both have a win and loss each.

Despite the defeat, Shakhtar boss Paulo Fonseca believed his side had caused the opponents more trouble than any side that had visited the Etihad Stadium this season.

"I'm disappointed with our defeat, but I'm proud of my team and their performance," Fonseca said. "Shakhtar showed great courage.

"We tried to play our football, to possess the ball. We were very compact. I've watched all City's games this season and no team caused them as many problems as we did."