The clash at Old Trafford is expected to be the biggest game of the season so far, with both Manchester sides boasting perfect records so far in the league. However United lost a significant number of players to international duty, after their late 1-0 win over Hull City.
Mourinho only has a handful of players to work with, with eight outfield players and goalkeeper Sam Johnstone remaining at the AON Training Complex, but with Sergio Romero, Marcos Rojo and Antonio Valencia returning just 24 hours before the clash, Mourinho admitted that it has made his preparations complicated.
He told MUTV: "We keep eight outfield players and Sam Johnstone as the goalkeeper," but was adamant that "We are going to lose the others and we don't know the way they train, the way they recover, the life they are going to have with their national teams."
Mourinho continued: "Some are coming back on the Monday before the Manchester City match but some are coming back on Friday. The day before Man City, we will still have Sergio Romero, Marcos Rojo and Valencia arriving."
He was firm in the belief "It will be a complicated process but we know it is like this when we have these players. And I would love all of the players to go because it would mean that everybody is being selected for their national teams."
Nobody is safe
Mourinho has already had a huge impact on the Red Devils, with significant players been brought in, exiting, or been dropped from his squad. One player that seemed unable to be dropped was skipper Wayne Rooney, but stated that Rooney wouldn't be afraid to drop his captain.
Mourinho concluded, simply saying: "I can take him out. It’s no problem for me to take him out, no problem for him to be out."
Manchester United will take on Manchester City at Old Trafford on Saturday, September 10 with kick-off at 12:30pm BST.