Primarily, I'd trust Masip as 2nd GK. We may have our doubts on Masip for now but he's sure as hell better than Pinto was and doesn't need to be focused on.
You have to remember that guys like Navas and Caballero play for defensive teams in Levante/Malaga. Being a GK behind a high line in a passing system like Barca's is different from having a wall of CB's right in front of you. They rarely move around or out of the 6 yard box, don't often face breakaway situations, and would need to be focused on for adaptation.
Also, we need to be confident on MAtS from the start. Better to focus on him and work with him as much as we can than spend all our time planning for any bad hypotheticals and whatnot. It's better to focus and develop our plan A to full potential than waste time on plans B and C. MAtS is the exact profile of a GK we're looking for in terms of style: He has height, claims crosses, organizes defenders, played in Buli against numerous quick counterattacks, and is great in a passing game. Nurturing that will keep us in aces with our GK situation.
That's why Masip is the perfect backup. He knows the system and fits the system so we can spend more time focusing on MAtS and other areas than vetting/prepping a bench player. He's a quality shot stopper and not crazy enough to dribble out to midfield.