The full-back replaced Claudio Bravo after the substitute goalkeeper was sent off during the 1-1 draw in Milan.
Bravo was shown a straight red for a sliding tackle on Josip Ilicic outside the box, having replaced first-choice keeper Ederson at half-time.
Guardiola said, "In this competition you know you have your chances and moments and you have to take it. But with the problems we have, we made a good first half. First half we were outstanding and second we suffered. In the last 15 minutes we had the issue with the new keeper."
Asked if Ederson miss the game against Liverpool, Guardiola added "I don't know right now."
Did the game change on the missed penalty?
"Definitely. Football is emotion.
"The second half we didn't do exactly what we should do. It was few chances conceded against one of the teams who create more. It was a perfect result away and we need one more point to go though.
"When we land in Manchester we will think about the next game in the Premier League."