Alexis was absent from the training session ahead of the game with Juventus, and Mourinho says the forward will play no part in the pivotal European encounter.
"The players you saw at training are the players available. Sanchez is not available," he told reporters.
Sanchez's absence comes amid rumours he could be set to force a move away from Old Trafford, with The Times claiming over the weekend that he is unhappy with Mourinho and that his agent is on standby to talk to interested clubs should they come calling in January or next summer.
Sanchez has struggled for form so far this season, scoring a single goal in seven Premier League appearances.
After being signed from Arsenal in January 2018 in a swap deal that saw Henrikh Mkhitaryan join the Gunners, Sanchez has made 26 appearances for the Red Devils.
He has scored just four goals and provided six assists in that time, with Mourinho using him sparingly.
Sanchez was left out of the squad to face West Ham earlier this season, and has been substituted numerous times.
Jesse Lingard, Diogo Dalot and Marouane Fellaini were also absent from the session, as the club prepare for the biggest game in their Champions League campaign so far.
Of course, the match with Juventus offers the chance for United to renew acquaintances with the iconic Cristiano Ronaldo, who has been enjoying life at his new club since a stunning move from Real Madrid.
Ronaldo has so far scored five goals and registered five assists in nine Serie A games.