City won for the first time in seven games in all competitions on Saturday, crushing West Bromwich Albion 4-0 in the Premier League, but Guardiola knows his former club represent a far greater challenge.
"I have never thought that we can't win a football match and I never will. Even when we lost 4-0," Guardiola told a news conference on Monday.
"It's difficult and we know we need to play almost perfectly to win - and if not, we will congratulate them and move on.
"Winning the last game helped a lot but the Champions League is much different. Tomorrow we have another chance against the best."
While Barcelona can secure progress to the knockout phase at the Etihad on Tuesday, City are in a battle with Borussia Moenchengladbach for second spot in Group C.
Another loss to the Catalans would leave their progress in real jeopardy.
"For them it's not a final, for us it is like a final," Guardiola, who is yet to taste defeat at home since taking over at Man City, said.
"There is just three games left. We dropped points in Glasgow (Celtic) and we have to recover those points."
City's cause is not helped by injuries with the right back area particularly worrying.
Bacary Sagna and Pablo Zabaleta are both unavailable, meaning Guardiola has to choose between Fernando, who played there for the first time against West Brom, and 19-year-old Pablo Maffeo.
"We have to think we've not got right backs," Guardiola said. "We have one who recently played there and one who is 19."
Striker Sergio Aguero will start, however, after being surprisingly left out of the away clash.
The Argentine scored twice against West Brom and looked close to being back to his best.
"When he is happy of course all the team is happy," Guardiola said.
(Reporting by Martyn Herman, editing by Ed Osmond)
