Last weekend, Ibrahimovic ended a seven-month spell on the sidelines as he came off the bench during the 4-1 win over Newcastle United after successfully recovering from a serious knee injury.
Mourinho has acknowledged that United needed "a second option" in attack, but the Portuguese has praised the influence of the veteran Swede who continues to work towards a first start of the season.
The 54-year-old told reporters: "He's not a player that you play only if the team is losing or only if the team is winning. He's a player that's easy to bring into the game, so normally he's going to have these periods of 15 to 20 minutes and, one day, we will feel that he's physically ready.
"He's always here, he's always positive, he's a good influence. It doesn't change the fact he's playing or not playing.
"That is important on the pitch because he's one more option for us and I think it's impossible for Lukaku to play every match, so we need this second option."