Thiago Silva's stoppage-time equaliser snatched a point for the Serie A side, who had also started the match in stunning fashion with Alexandre Pato netting in the opening 30 seconds before Barca turned the game around through goals from Pedro Rodriguez and David Villa.
Guardiola could not fault his team, though, telling www.fcbarcelona.cat: "They worked hard in terms of structure and play. Milan couldn't get across the halfway line.
"We were Barca. We played as we always have done - trying to play football, shooting... everything we were supposed to do to win.
"We managed to hold Milan back. It was a more than acceptable performance. Only little things were missing, and they did score in the first minute.
"They only attacked two and a half times, but these things happen in football.
"We have to be more careful and we'll be looking to correct things."
He added: "We didn't lose and there are five games left."
Barca let a 2-0 lead slip to draw 2-2 with Real Sociedad in their second Primera Division game of the season at the weekend, but Guardiola remains fully behind his squad.
"I have no doubts about these players, you know that," he said. "If these players have shown one thing it is that nobody works harder than them. I sense that they know how difficult this is. It gets harder every time."
Goalkeeper Victor Valdes agreed with his coach, that Barca must learn lessons from Tuesdayt.
He said: "It was not a result we liked and the draw certainly came as a disappointment.
"We need to learn and even more so in dead-ball situations. We have to analyse and learn from our mistakes and make sure they don't happen again.
"We need to pick ourselves up quickly because we have a league match to come [against Osasuna on Saturday]. This has only just started."
Valdes also expressed his sadness for midfielder Andres Iniesta, who will be sidelined for around a month after suffering a hamstring injury on Tuesday.
"It was the worst news of the day," he said. "It was nasty shock but I hope he recovers as quickly as possible."