He HAS to start against Arsenal at the Camp Nou
I really thought Mascherano should have started in the 1st leg and was saying so before the game, wanted Busquets as CB and Masch DM but I'm not 100% convinced on him starting 2nd Leg as that's when we'll be controlling the game much more than the 1st leg and we might need Busquets in DM to play our possession game?
Masch would have been great for breaking up Arsenal's plays in the first game and running and sliding at their players but Busquets is probably better on the ball and I don't expect Arsenal to have much of it in the 2nd leg.
Having said that, I think we should be giving Mascherano and Affelay as much game time as possible as with more experience and confidence they could really be great cover for our usual three. Both offer something different to our usual - Masch is faster and 'harder' and Affelay is maybe more direct and can take long shots.
Though it's hard to break up the unbeatable triangle of Busquets - Xavi - Iniesta !