The reason Douglas and Vermaelen got to play against Depor is because they had played close to zero minutes all season in total, unlike Montoya, so Lucho wanted everyone to be part of the championship.
And how does any of that change how he'll feel if he's our consistent starter for six months? What, you think he'll say "Last season you didn't play me well and I want revenge! I'm leaving!" the moment the transfer window opens up?
I can ask you the same question, why would Lucho all of a sudden think Montoya is consistent starter material just because his other option is Douglas?
When the transfer window opens after those 6 months Barça will be looking for a new right back.