Mourinho fears he will be without up to seven players for the Old Trafford clash United must win to keep alive their hopes of securing a top-four finish.
United are currently without injured quartet Phil Jones, Chris Smalling, Marouane Fellaini and Paul Pogba, while Ander Herrera and Zlatan Ibrahimovic are suspended.
Skipper Wayne Rooney has been out with a knee injury for the past three weeks and there is no guarantee he will be fit by Saturday, although sources have indicated the 31-year old is ready to return.
But Mourinho may be forced to rush Rooney back to plug a gap in midfield, with the enforced loss of Pogba, Fellaini and Herrera leaving him light there.
Michael Carrick remains Mourinho’s only fit central midfielder, a position Rooney may have to occupy on Saturday if he is fit and able to start.
Rooney last featured in midfield last season under former boss Louis van Gaal, although versatile defender Daley Blind is also able to operate there if required.
Mourinho’s side are currently fifth, four points behind fourth-placed Liverpool, with two games in hand on their arch rivals.
Midfielder Juan Mata admitted United the upcoming month will be a decisive one for Mourinho and his players, with nine games to play.
Mata said: “It’s not a random list of teams, it’s the list of the eight teams we’re going to face in a frantic month of April.
“Since we have to play twice against Anderlecht [first and second leg], it makes a total of nine games for us the next month.
“It’s not a complaint, it’s just a reality and we have to adapt ourselves to it, in order to give the best we have in the decisive spell of the season.
“I see these games as a challenge to show all the good things we’ve been working on throughout the season, and I hope that we can get good results too.”