Mascherano as a CB is always a certain risk. And I think against Bayern, a team who predominantly rely on aerial success to score, Mascherano will be exposed. Nothing against him, he did great against Atleti with Mando, but Bayern's attack is too sophisticated aerially for Mascherano to deal with. It's too big of a risk and simply not a necessary one with either Mathieu or Bartra available and in form.
You guys are too harsh on Mathieu, he can do this job very easily and with minimum fuss. Their strength is precisely why we brought him into the squad, and as Dr Pepper stated, he was good against RM, a team who press relentlessly as well.
Honestly, if any of you guys know my track record, I'd go Barta-Piqué as the CB pairing in both ties but that's just me. Don't expect Lucho to do that.