Paulo Dybala's first-half goal gave Juve a vital victory at Old Trafford in a game that they dominated for large parts, leaving them five points clear at the top of Group H.
Mourinho resisted the urge to make a single change during the game, despite having a full bench available to him and, Paul Pogba's effort against the post aside, his side struggling to create opportunities in front of goal.
Defenders Eric Bailly and Matteo Darmian, midfielders Fred, Ander Herrera and Andreas Pereira, and forward Tahith Chong were all options, but Mourinho did not feel inclined to introduce any of them in an attempt to change the game.
"Well, you have to bear in mind that Alexis Sanchez was in the stands, Jesse Lingard was in the stands," Mourinho told reporters.
"Fellaini may or may not have been in the stands – he might have been at home I didn’t see him. And the only really attacking options I had on the bench was a young 18-year-old lad who has never played in the first team before.
"And I didn’t think, given the dynamic of the game, it would have been appropriate to put him on.
"You can’t expect a young kid coming on, making his debut in a game like that, to be able to give me something like scoring a goal.
"So, basically, that’s why I didn’t make any changes and just stuck with the players we had out on the pitch."
United travel to Italy to face Juve in their next Champions League encounter, but Mourinho has already admitted that overhauling the Italian giants may be too big a task for his team.
"I have to say they [Juve] are a different level and if people don't want to accept that it's their problem," Mourinho added.
"They are a different level of stability, experience and knowhow and at the base of the team they have Bonucci and Chiellini, it allows them to play with the freedom in attack, creating and losing the ball, 'no problem, we are here'."
United are back in Premier League action on Sunday when they host Everton at Old Trafford as the Red Devils look for what would be just their second win in eight matches.