Ahead of the semi-final second leg against Celta Vigo on Thursday, Mourinho had rested first-choice players in Sunday’s 0-2 Premier League defeat by Arsenal.
The News Agency of Nigeria (NAN) reports that the loss meant United’s hopes of finishing in the top four were in jeopardy.
Manchester United lead Celta 1-0 from the first leg in Spain and victory in the final against Ajax Amsterdam or Olympique Lyonnais would seal a place in next season’s Champions League.
“The situation is simple — 17 matches in seven weeks with 16 players is impossible.
“It was not a gamble, it was a simple decision. A decision based on common sense, Mourinho said”.
NAN reports that United lost several key players to injury in April, including leading striker Zlatan Ibrahimovic and defender Marcos Rojo.
“When we lost these players in a period of seven matches in seven weeks it was the impossible job. It was not a gamble but a consequence of the situation we are in.
“To be in the final would be very important to fight for the only trophy the club has never won before.
“We will do tomorrow (Thursday) what we have done since the start of the season which is to fight hard,” he said.
If United fail to win the Europa League and finish outside the Premier League’s top four they will not play in Europe’s most prestigious club competition.
It will be the second season in a row this would be happening to them.