Whilst I'm sure watching Barcelona dominate some lower league team in La Liga might be quite exciting- watching them play against a team who know how to defend is like watching paint dry.
But he was putting his arms up and turning to the side in an effort to avoid getting in the way of the ball. There has to be intent for it to be an offence.
(Just putting the other side of the argument out there - I was surprised the goal was given, but I don't think it is so clear)
Just saw a replay, it bounced of a barca player's arm onto the milan player's arm. If this was american football those two fouls would offset. I'd say given the unexpected deflection, an the milan player moving to avoid the ball (prior to said deflection) it would be harsh on him, and harsh on the barca player who had turned away to avoid getting a ball in the face. After the replay, I think the ref was probably right to let it go.