The Red Devils, who get their campaign underway on August 10 against Leicester City, will be without 12 first-team players, who have been given three weeks' holiday.
Paul Pogba, Romelu Lukaku, Marouane Fellaini, Jesse Lingard, Marcus Rashford, Phil Jones, Ashley Young, Marcos Rojo, David de Gea, Nemanja Matic, Victor Lindelof and new signing Fred will all be absent.
Mourinho hit out at the decision to start the English top-flight season so soon after the World Cup finishes and revealed the impact on his team selection.
"We cannot start the Premier League with our players that were in the World Cup, which is a shame for the most important club competition in Europe," the Portuguese told RT.
"Three weeks, which I think is the minimum that their body and soul needs, and which is the minimum for them to return and think about football again.
"To bring them back before that would be, 'Okay, I have you for the first match of the season, but maybe I don't have you in three or four months.'
"I think three weeks is the minimum they need, which means that they're going to be back in the week when the Premier League starts, which means they're not ready to start the Premier League, which is I think difficult to understand, where we could have one more week to prepare everybody."
United's first pre-season encounter takes place on July 19, a friendly against Mexico's Club America in Arizona, United States.