United's preparation for the new campaign has been badly affected by injuries and the absence of a number of players whose summer break was delayed due to the World Cup.
Ander Herrera and Nemanja Matic are among six players ruled out of the game with injury, with captain Antonio Valencia, Marcos Rojo, Sergio Romero and Diogo Dalot also sidelined.
Detailing the problems in his pre-match press conference, Mourinho said (via ManUtd.com):
"Unavailable is Romero - he had a surgery after his injury before the World Cup. Valencia - he was injured in pre-season. Rojo - he was injured in the World Cup. Dalot - he had a surgery at Porto by the end of April and is not yet ready. Herrera - injured against Bayern. Matic - injured in the World Cup and surgery in Philadelphia a couple of weeks ago. I think that's it."
As far as the rest are concerned, Paul Pogba, Jesse Lingard, Ashley Young and Marouane Fellaini are very unlikely to feature as they only returned to training on Monday.
"They returned on Monday. They trained on Monday, Tuesday, Wednesday and today, so you can imagine that they're not really fit and ready, but we have needs," said the boss of the quartet.
Others who were also at the World Cup until the final weekend, like Marcus Rashford, Romelu Lukaku and Phil Jones, have more of a chance of being involved after returning to training earlier.
"The players with a proper pre-season are fit and they are strong. For example, Alexis Sanchez had a good pre-season with lots of matches and lots of minutes," Mourinho explained.
"He's more than ready but you cannot have the players who arrived only last Monday after the World Cup semi-finals or final and three weeks' holiday. You cannot expect them to be fit."