Hmm that is probably the most clinical defeat Barca has suffered since Inter beat them 3-1 in 2009/10 champions league semi-final, coincidentally also @ the san siro. Worst noticeably poor game Messi has had in years also, can't remember when last i saw his touch so awry.
Most of world football has accepted you can't beat Barca playing football (except the couple times Real did last year). Its always the park the bus + clinical counter attack that tends to upset them.
Sometimes it works (inter 2009/10, madrid at times under mourinho, man utd & chelsea in past c-league battles, couple defeats in la liga over the years). But usually it doesn't work.
However celtic & Milan have done it to them twice now this season, which probably will concern them. They will do well to reach the quarter-finals after this.