City ultimately fell to a thumping 4-0 defeat in Catalonia, Messi bagging a hat-trick, but Guardiola insists that his side were in the game before going a man down.
The former Barca boss told reporters: "At that level it is hard, but until 10 against 11 it was open and we were competing against a team with a big personality. But after the card it was over.
"Every game since Celtic it has been the same, against Everton it was the same with an own goal and two missed penalties. We need to change the atmosphere.
"We were there, we were in the game. We were pressing, we had the possession and we made the chances. We did it but we didn't take the chances and you have to do that to win the game."
Up next for Man City is a Premier League home game against Southampton on Sunday, October 23.