Sterling (hamstring) has not featured since the first game against Basel, while Fernandinho is recovering from a hamstring strain sustained in the Carabao Cup final.
The boss also gave an update on Benjamin Mendy following the defender's return to full training after a lengthy spell out with a knee injury.
Guardiola said: "Ferna is much much better, as is Sterling. Maybe tomorrow they won’t play but they may be ready for Stoke City. Mendy is back but he’s been out for six monhts, we have to be careful."
City won the first game thanks to goals from Ilkay Gundogan (two), Bernardo Silva and Sergio Aguero, but the boss says there will be no room for complacency.
"Tomorrow we start again from zero," Guardiola said. "In knockout games you have to look at what you have done in the first game.
"We have to be focused not to make a mistake. If you are focused on what you have to do on set pieces and you control the game, you will go through.
"The Champions League is something special, you have to be focused. But in football, anything can happen."
He added: "We have won more games so we have confidence. Last season were were more up and down.
"This season we have won a lot of games so we have more confidence and are at a more stable level. Always there is time to learn and tomorrow is good test to learn how to handle a good moment.
"How we control the game, how we create. Maybe tomorrow it’s more important nothing happens but we don’t concede, set pieces or counter attacks. We play simple. It’s a good moment to see how we can handle the situation for our future."