Ibrahimovic returned after a seven-month lay-off during United's win over Newcastle last weekend and continued his road back to fitness coming on as a substitute in the midweek Champions League defeat at FC Basel.
The 36-year-old hasn't started a game since injuring his knee back in April and manager Mourinho is keen to continue to take things slow.
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"No, not ready (to start)," Mourinho said of the Swede ahead of this weekend's game with Brighton. "Not ready. Step by step, minute after minute, match after match, I think it is easy to play him if the team is losing - you need him on the pitch for the last 15 or 20 minutes.
"If the team is winning, you need him on the pitch for the last 15, 20 minutes because his qualities are adapted to both situations."
Paul Pogba is set to start again while Marcos Rojo, himself a long-term injury absentee, will also feature against the Seagulls at Old Trafford.
The return of such players is important at a crucial stage of the Premier League campaign, with second-place United facing Brighton, Watford and Arsenal in quick succession.
"We have four competitions to play and let's go step by step, it's the only thing we can do," added Mourinho, whose side enter the weekend eight points to leaders Manchester City. Match after match, Champions League finished against Basel, we switch off and move to the Premier League.
"It is an interesting week because I think is the first time this season where we have three Premier League matches in one week."