Deputising alongside Fernandinho at centre-back, the Spaniard was forced off in the Champions League Group C clash against Atalanta with what seemed to be a hamstring strain shortly before half-time.
Speaking to the press after the game, Guardiola explained the midfielder will be assessed on Wednesday with the expected timeframe of absence to cover between ten days and three weeks, depending on the severity.
"We do not yet (know what injury is)," he stated. "Tomorrow (Wednesday), we will know.
"Hopefully, it will be ten days if it's not broken. If so, then three weeks.
"It looks like his hamstring."