The visitors could not have wished for a better start, breaking the deadlock right in the opening minute through Alexandre Pato, before Barca turned the score around with goals from Pedro and David Villa.
And just when the hosts thought they had secured the victory, Thiago Silva silenced the Camp Nou with an injury-time header that forced the two clubs to share the spoils.
After the game, Guardiola talked about AC Milan's stuffy defensive performance, saying: "It is never easy to attack when their midfielders convert into centre-backs.
"If they have to defend in the penalty box, they do it. Milan have a lot of merit for this result.
"They reached our goal two-and-a-half times and they got a point. European teams like this reinforce even more what we have done."
The 40-year-old played down the negative result in their opening Champions League game and stressed there is a lot left to play in the competition.
"We had a good match and if we consider the 90 minutes, we see we had a good match, we closed down Milan, though there are things we have to improve," Guardiola noted.
"This is the first of six matches and we have five games ahead."
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