The Brazilian was introduced as an 85th minute substitute at the Etihad Stadium, but was forced off before the end with a leg injury that could keep him out for up to six weeks.
City’s start to the season has been plagued by injury problems and with a busy period of fixtures on the horizon Guardiola admitted it is a set back to lose the 35-year-old.
“It is bad news,” the manager said after the game.
“It is a problem in his leg. I think he will be out for four to six weeks. It is a big blow for us, with the amount of games.”
Guardiola saw his team come from behind to secure all three points against a stubborn Porto side in the opening game of our Champions League campaign.
Sergio Aguero, Ilkay Gundogan and Ferran Torres were on target after Luis Diaz had fired the visitors in front and it was a performance which pleased the City boss, who was impressed with how his players managed a physical encounter.
"I’m happy with all the game," he added. "We knew how tough it would be.
"Alongside Benfica they are the best team in Portugal. They played so defensively with five at the back, we had to be patient.
"In the second half we played really well. We didn’t conceded counter attacks or set pieces where they are so strong. We didn’t allow them to run, where they are dangerous.
“It was an incredible performance from us against a really tough team.
“It is nice to win games suffering against a really good team. I am so pleased with the last two games.”
The duo are expected back in training before the trip to the London Stadium and it is a welcome boost for the manager who says he will need all his players during this busy period.
“But I don’t know how they will be tomorrow and the next days. People come back but we’ve lost another.
“It is important for people to come back. We go to London in three days and then after three days we go to France to play Marseille and after three days we go to Sheffield.
“The schedule is so demanding. We need all the players.”
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