I reckon there is a priority order in place but then there is no accountnig for someone like Rodri doing what he did. However good he is in his position I can't believe he is anywhere near the top of the list of penalty takers in our team. He's taken one in the past for us and ok scored it but he was lucky and even Pep said after it that it was a poor pen.Taking a penalty when your already 2 up is not the same as a last minute against high ranking opponents in the last minute to win the game.
Mahrez has often bottled the big ones - Liverpool away, West Ham away (good save) for example in what seem pivotal games.
As stated many times the players taking pens and the priority order should be nominated prior to kick off and only changed at the captains say so when having to manage an on pitch issue.
On Sunday the whole thing looked a bit unprofessional.