Mourinho just explained it in the post match interview.

He has 3 designated Penalty takers: Pogba, Zlatan and Rooney.

Paul and Wayne have decided that Ibra is the first penalty taker. And whenever he doesn't want to (for some reason), they will take it.

So:

1. Zlatan

2. Pogba or Wazza

Interesting to see he lets the players (these 3) decide who will take them, considering they all want their goal record to improve. And notable that Mata or Martial isn't one of the options.