The 35-year-old former Swedish international has found the net 14 times in all competitions for United this season after he joined from French champions Paris Saint-Germain on a free transfer.
Ibrahimovic has also played in more matches than any of United's other strikers this season, featuring in 24 of their 26 games so far, and will start up front in the Premier League clash with in-form West Bromwich Albion on Saturday.
“Is it almost impossible to leave him out? Yes, it is,” said Mourinho at the eve of match press conference on Friday. “As a striker he is in one of the impossible positions where you cannot hide.”
“He is phenomenal and plays again tomorrow and then one week to rest.”
“I am not surprised at all. I knew from the physical point of view he could resist but obviously he cannot play 60 matches.”
Mourinho, who enjoyed a good relationship with Ibrahimovic at Inter Milan when he was in charge there, said he would need to rest him but he was such an integral part of the side it was hard to omit him.
“I must give him a rest. I must leave him out a game,” said Mourinho.
“He was suspended against Arsenal. So that game he did not play.”
“In some matches in the Europa League he was on the bench, also against Northampton (in the EFL Cup).”
“So now and again I will find a way to give him a rest.”
“But as our target striker we can see he is the only one. And the way we normally play we need him.”
Mourinho said Ibrahimovic is also a great example to United's youngsters with the manner in which he conducts himself in matches and on the training ground.
“He is a great example in every aspect, he is very professional,” said the 53-year-old Portuguese.
“Great also with his family life and on keeping his family life the most positive and private.”
“He is a great example in everything and when you have him and Michael Carrick at 35 the kids could not have better examples.”