Sure it was a bit of an anomaly, and still required plenty of luck, but with Barca's depleted defense the chances of scoring two goals against them are certainly not "microscopic". They are literally a complete shambles when it comes to defending corners and freekicks. Look at how many times they've come back from behind in La Liga this year against inferior teams.
If you play 10-20 games against Barca in that style you would probably still lose more than half of them, but Chelsea and Celtic (two teams with vast differences in quality, as you said) have shown that deploying those tactics in one-off games or cup ties gives barca a lot of problems and can effectively bridge the talent gap between the two teams. The fact that Chelsea and Celtic are so different and yet achieved almost the same results using the same tactics just proves my point.
Anyway, enough about that. I'm excited for Chicharito getting some playing time and goals in Man U's new setup. He's got to be one of the best goalscorers in the premiere league and is finally getting his due now that Rooney's taking on a bit of a deeper role. He kind of looks like a muppet though.
Poor, poor Arsenal as usual...