The 35-year-old Swede has missed only one Premier League game for United this season scoring nine goals in those 15 matches.
"Tomorrow is a last effort and then they will have a week without football," Mourinho said. "That week is a week that my players need, especially a guy like him - a striker.
"Sometimes in other positions you can protect yourself a little bit more with experience and with position but a striker is one of the impossible positions where you cannot hide.
"So he is doing phenomenal. Tomorrow he is going to play again but obviously he cannot play 60 matches. I must give him a rest.
"But as our target striker we can say that he is the only one and the way we normally play we need him."