The Manchester City full-back visits a knee specialist in Barcelona after suffering the injury last weekend against Crystal Palace.
Guardiola said after their Champions League win over Shakhtar Donetsk on Tuesday: “I think it is going to be a major setback”.
When asked if he would consider looking for a replacement left-back during the transfer window in January, Guardiola said: “We will see in January.
“We thought next summer about looking for another left-back, another replacement, but we will see what we do.
“We have alternatives in our squad – Danilo, Fabian Delph, Fernandinho, can play there so we will see as we go along.”