Masche and Mathieu were the failing CB paring vs PSG. I think Lucho will learn from that mistake and play Mathieu at LB and with Pique Masche or Pique Bartra at the back. That will depend on Busi if he's healthy, if he isn't Masche in mid.
Ya it's looking like Mathieu should start at LB to give the defense more stability against Madrid. Look, against most La Liga teams the top CB pairing should be Mathieu and Masche. They are the ones who make the least mistakes right now.
BUT, against the top teams Mascherano unfortunately is a liability and it's not anything to do with his play. He is just too short. It's generally not a problem in the run of play but every top team will undoubtedly get several corner and free kick chances to cross it into the box. Against other teams it's fine with Masche at CB cause the opponent on,y gets a few set piece opportunities per match. Barca can weather that that storm. But the big teams will get 10+ set pieces per match and Masches height puts us a a serious disadvantage.