The Algerian worldwide is widely viewed as one of the top attacking left-backs in the world and has no intention of signing a new deal at San Paolo, where City will travel to on November 1 having been paired with Maurizio Sarri's side in the Champions League group stage.
"Benjamin will undergo surgery tomorrow in Barcelona, because he has a cut in the front cruciate ligament of the right knee, and everyone in the club wants to recover quickly".
City splashed out £52million to bring Mendy to Manchester from Monaco during the summer.More news: England suspends Stokes, Hales
Delph, who has filled in for Mendy at left-back in recent weeks, said he and his team-mates were keen to repay the gesture.
Delph played at left back in City's 2-0 win over Shakhtar Donetsk in the Champions League on Tuesday.
"I know how hard these situations are".
"We will see in January", the City boss added.More news: Here's Where You Can Get Free Coffee On National Coffee Day
To make matters worse for Pep Guardiola and City, star striker Sergio Aguero has reportedly been involved in a auto crash after attending a concert in Holland.
Meanwhile, Chelsea head coach Antonio Conte said the hamstring injury sustained by striker Alvaro Morata in the first half was "not serious", although he is expected to miss global duty with Spain.More news: Two separate expected volcanic eruptions spur evacuations on Bali and Ambae