The Glasgow giants ended City's perfect start to life under Guardiola by forcing a 3-3 draw at Celtic Park, having lead on three occasions.
In each trailing situation, City managed to equalise but the boss believes perhaps their performance was good enough to claim all three points had they not conceded in the 3rd and 47th minutes.
"It was difficult for us. I'm happy with the reaction from the players, it's not easy to score three goals away," said Guardiola.
"I don't know how many chances they had after the third goal. We should have won it. It's disappointing to concede at the beginning of each half, but that's football.
"I know how difficult it is here and they started so strong. The second goal was unlucky and I don't remember any action for them after they scored their third goal."
Moussa Dembele opened the scoring early before Fernandinho responded on 11 minutes. An own-goal from Raheem Sterling in the 20th minute gave Celtic the lead before the winger atoned for his error by levelling six minutes later.
Talented French striker Dembele grabbed his second early in the second half but Nolito ruined the Bhoys party by tying it up on 55 minutes in what was a dramatic night of European football at Parkhead.