Yeah it is debatable, and I agree we should take it elsewhere next time.Thats debatable to say the least which is why you have people still arguing over their treatment to this day. Anyway this is not the Martial or Mkhitaryan thread so lets not derail it further.
As for Mourinho not rotating Zlatan its simply his preference because Rashford and Martial didn't do too badly last season and we were reasonably successful. So why does Mourinho refuse to use them in the smaller cups, particularly Rashford to maintain Zlatan's freshness for the more important league competition or even sub him off when we are comfortable?
As for Zlatan, he has started rotating him lately. Earlier in the season, I think the grounds for having him on even in lesser cups was to have the starters getting as much game time as possible to make them gel.
There should be no excuse now though not to give Rashford more starts in cups. I believe we should be fine without Ibra in those kind of games.