The 36-year-old made his return from an eight month injury lay-off in United's 4-1 win over Newcastle United last Saturday.
But speaking ahead of United's Champions League trip to Basel, Mourinho said the Swede still needs time to get back to full fitness.
"Zlatan goes on the bench tomorrow," said Mourinho.
"He needs to play and he is useful for us.
"If we are in a winning position, like we were against Newcastle, we need him on the pitch, because he keeps the ball, he keeps possession. He drops into areas where he gives stability to the team
"If the team is not winning, his presence on the pitch will be important and it will be positive for him to play with (Romelu) Lukaku.
"Normally, he is going to play, and I would say it will be a little bit more comfortable for him to play a bit more than he did the last match.
"Instead of 15 [minutes], I would say 20, 25. But [he is in] no condition to start."