Fernandinho sat out surprise back-to-back defeats against Crystal Palace and Leicester which saw City drop seven points off Liverpool's pace at the top of the Premier League.
And the injured Brazil midfielder is not certain to face Southampton on Sunday despite training on Saturday.
"It is not necessary to lose three games from four to say how important Fernandinho is to us," Guardiola said. "It is a specific position, we don't have his qualities again. He is an incredible player."
City tried to sign cover in the form of Jorginho in the summer, but the Italy midfielder opted to follow his Napoli boss Maurizio Sarri to Chelsea instead.
"We tried but when players don't want to come what can we do?" Guardiola said.
"I never in my career with Barcelona or Bayern Munich, I never complain about what they try to do, the clubs. They do their best because they want to win too. They try to provide me with the best team possible and they did it."
But though it is a problem City have sought to address in the past, Guardiola once again indicated there would be no signings in January.
"I think the answer was clear two times about this issue," he said.
Guardiola has never been one to compromise on his approach to the game, and insisted there was no reason to change anything now.
"We have belief," he said. "We have the belief when the situation is win or lose. The belief is more about what we are, no matter if we win or lose.
"We have to come back to win games if we want to still be there for trophies but the belief is always there.
"Why should I not believe with these players and what we have done in the last 15 or 16 months. They are absolute heroes for me."
And the Catalan said it was up to him to find the answers.
"I know my players perfectly," he said. "If you believe I am going to doubt one second about them you are completely wrong. For me it is incredible what they do, what they have done and what they are doing in every single game.
"First of all I sit in front of the mirror myself. I never point the finger, never. I sit in front of my mirror and ask what I can do to help because I am able and I am willing to do that."
While Guardiola may insist there will be no new faces in January, City expect to lose teenage midfielder Brahim Diaz to Real Madrid despite hopes he would commit his future to the club he joined from Malaga in 2013.
"I don't have any new information," Guardiola said. "I know they are in conversations but I don't have new info on what the situation is. Today he trained with us. I spoke in the past what is my opinion."
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