Guardiola said: "Congratulations to Manchester United for winning the FA Cup. I think my game plan was not good.
"The second half was much, much better. We were more intense, in part because we were 2-0 down and had nothing to lose. It was a tight game.
"We gave away the first goal, they made a good transition for the second so credit to United for the second goal. We tried. We had clear chances in the second half which wasn't easy because they had man marking and defended the pockets but we had the chances and unfortunately we scored a goal a little bit late. Maybe it could have been different but the team was there like all the time when we lose.
"Disappointed today, it's normal teams can lose finals but this season has been extraordinary fighting for all the trophies in a good way. We will rest and come back next season."
On United manager Erik ten Hag, he added: "They have to take a decision. He's a lovely person and an extraordinary manager. Winning the FA Cup is important for them like last season it was for us."