Zlatan Ibrahimovic and Henrikh Mkhitaryan were both named on the bench for the match, which the Red Devils won 2-0.
Mourinho reserved special praise for Marcus Rashford who opened the scoring and caused problems for the Blues’ backline throughout the 90 minutes.
Rashford latched on to Ander Herrera’s inch-perfect lobbed through ball to slot past Asmir Begovic put United ahead in the first half.
Herrera turned scorer after the break, his deflected effort flying past the Bosnian to double the Red Devils’ lead.
“Maybe we didn’t ‘rest’ players ahead of Thursday, we just chose the team we thought was the best team,” Mourinho told Sky Sports.
“We went to Stamford Bridge in the cup with same tactics, the game was totally controlled with 11 players, only the decision that made us play with 10 men for the second half and 10 minutes in the first half, gave Chelsea a chance to be dominant.
“We knew that playing this way would be very difficult for them.
“Marcus Rashford played fantastically well. He had a very good game at Stamford Bridge in the cup, exactly the same chance to score, because he is faster than their defenders.
“Today he scored and it gave the team more confidence, more stability.”
Rashford pointed to the discipline of the performance as the key to victory, and played down the absence of Ibrahimovic.
“Today took a lot of discipline to carry out the game plan and actions,” he said.
“It’s not so much stepping up to the plate in Ibrahimovic’s absence, everyone is together and we grafted to put the work in to get the result.
“It was a clever pass from Herrera, we made eye contact and he put the ball exactly where I want it. It’s something we need in this team.”