Ibrahimovic has re-joined Manchester United on a new 12-month deal - just four months after undergoing career-saving surgery on a serious knee injury.
The Swedish superstar is continuing his rehabilitation and will not be available until the start of 2018.
But Mourinho has warned him he will have to fight for his place in the team like everyone else once he is available again.
The United boss said: “Zlatan knows me - and he knows that I play the players that I think are the best for the team.
“I always did that in my life, so if he comes here and he proves that he is the best, he plays. If the other ones don’t give him a chance to prove that, that’s life.
“I try always to be honest with my players and with my team. I try. Maybe sometimes I do not do the correct things. But I always try to do.”
Ibrahimovic has been included in Manchester United’s Champions League squad but Mourinho insists the former Barcelona ace will play no part in the group stages.
Mourinho, whose side host Leicester today, said: “I’m not thinking Zlatan to play any part of the group phase, hopefully he can play in the knockout phase but for that we need to finish top two.
“I don’t want to think about it, I don’t even ask or speak about it with the medical team.
“He will be an extra man for us in the second part of the season. When I say second part, we say January after Christmas, when the transfer window re-opens - but I have no idea at all.
“But for the player it would have been very easy for him to say goodbye. Because he had success at Manchester United.
“But he doesn’t do that, he wants to follow his other dreams and play football at the highest level.”
Mourinho, meanwhile, insists Wayne Rooney deserves the ultimate respect for having had such a successful career with England.
Rooney has retired from international football and Mourinho added: “He told me the night before. I am nobody to agree or not agree. I just listened to his reasons and I understand his reasons.
“I just realise, like many other people when he retires, how many matches he played, goals he scored, the football he represents in this country. Many congratulations for his career with the Lions.”
