City, second in the Premier League, face leaders Liverpool on Sunday who they trail by a point but the Spaniard said the focus remained on reaching the Champions League quarter-finals.
"In football, details make the difference. Everything can happen," he told reporters on Tuesday. "We concede a goal in the 85th minute ... concede two, we have to go to extra time.
"I have a lot of respect for Copenhagen. They defend, they have a good system... they're really well organised," he added.
Guardiola cited the example of RB Leipzig, whom City beat 3-1 on the road in the group stage but then went 2-0 behind at home before fighting back to win 3-2.
"Two mistakes, zero two," he said. "If we (conceded) another goal we'd be second in the table."
City striker Erling Haaland (23) echoed his manager's sentiments.
"I haven't thought of (the Liverpool game) until you asked me now," he said.
"I am focused on Copenhagen. It's going to be a huge game against Liverpool, we have to be ready for the game. We have to be brave and play at our best."
Guardiola said the team selection for the Liverpool clash will depend on the players' fitness after the Copenhagen game.
"Selection depends on the players, on how they feel. The competition is so important because in football everything can happen and you have to be aware of that," he said.
