"So we will have to be very careful and stay very focused to not allow that to happen."
City and Barcelona are strong favourites to progress from Group C but Fernandinho believes the outcome of the four-team league is far from certain.
"The Champions League is always special, you always have to be careful, you always have to think that you cannot make mistakes. There's no relaxing, otherwise you'll pay for it," he said.
The Brazilian has flourished more than most under Guardiola's early guidance, transforming into a pivotal member of the City midfield during their perfect start to the new campaign.
And his presence will remain even more of a boost for Guardiola in the absence of Kevin De Bruyne, who limped out of Sunday's 3-1 win at Swansea with a hamstring injury.
He is reportedly set to spend up to four weeks on the sidelines.
Meanwhile, Rodgers has a goalkeeping dilemma to resolve before Manchester City turn up.
The Scottish champions shipped seven goals at Barcelona and Rodgers needs the safest pair of hands he can find to keep out some of the most lethal finishers in English soccer.
City have scored more than anyone in the Premier League this season.
Unconvincing Dutch goalkeeper Dorus de Vries was substituted at halftime due to injury in Celtic's 6-1 Scottish Premier League win over Kilmarnock on Sunday, with Scotland international Craig Gordon the replacement.
"Dorus strained his chest when he landed. He could have played on but we had Craig there fit and capable and I didn't want to take any risk with it so we will see how Dorus is [over the next couple of days]," Rodgers said.
The Northern Irishman said he would have no qualms about fielding Gordon against City, who beat Borussia Moenchengladbach 4-0 thanks to a Sergio Aguero hat-trick.
Tomorrow's game is the first between the two sides in European competition, but Celtic have several ex-City players in their squad including defender Kolo Toure, Scott Sinclair and on-loan winger Patrick Roberts.
