The Citizens were beaten 2-1 by Manchester United at Wembley in May, a result that kept boss Erik ten Hag in the United job at the time.
Guardiola, who was disappointed at how his side played in the game, admitted their Premier League title winning celebrations had impacted their display.
The Catalan said: "This is the news, not that we lose, but how long it has been since we lose.
“Of course, with the FA Cup final we were a little bit hungover, but it is good news."
Speaking about their Carabao Cup loss to Tottenham this week, he added: "It's my mistake - the game plan was not good. The players know it, the reason why. Tactically, it was not good. I had the feeling it was not good today. You plan a game for different positions, but didn't work - it's as simple as that."
The City boss also said: “In football and in life, you win and you lose. We lost in the right way, especially in the second half, as they (the players) didn’t give up and kept going.”