The outspoken 35-year-old arrived at Old Trafford to much fanfare in the summer given his unerring knack of finding the net and medal-laden CV.
Few expected Ibrahimovic to have such an impact at United, though, given his advancing years and their punishing fixture schedule.
The striker has featured in 24 of United's 26 matches this term but shown few ill effects, scoring his 14th goal of the campaign in Wednesday's 2-1 win at Crystal Palace.
Asked if it was almost impossible to leave Ibrahimovic out, Mourinho said: "It is, it is.
"But tomorrow is the last effort and then they will have a week without football and that week is a week that my players they need, especially a guy like him, a striker.
"Sometimes in other positions you can hide yourself a little bit more, you can protect yourself with experience, with position.
"As a striker, I think it's one of the impossible positions where you can hide.
"He is doing phenomenal
Tomorrow he is going to play again, then one week to rest."
The break between the trip to West Brom and Boxing Day clash with Sunderland will be welcome respite for a player Mourinho will handle intelligently during the festive period and beyond.
"I am not surprised
I am not surprised at all," the Portuguese said of Ibrahimovic's impact.
"I knew even by the physical point of view he could resist, but obviously he cannot play 60 matches
I must give him a rest.
"I must let him out again
He was suspended against Arsenal so that game he didn't play.
"In some matches in the Europa League he was on the bench
He was on the bench also against Northampton, so now and again I will find a way to give him a rest.
"But as our target striker, we can say that he is the only one and the way we normally play we need him."
Ibrahimovic could be joined in the starting line-up at West Brom by Chris Smalling after the England defender recovered from a foot complaint.
Out since October 23, his return is a welcome boost to a defence without Eric Bailly due to a knee injury sustained in midweek.
"I don't know how long," Mourinho said when asked about the severity of the problem
"By the movement on the pitch we thought meniscus but no
It's a question to see the evolution.
"Hopefully we have him for these last two matches at home."
Bailly will join Henrikh Mkhitaryan and Luke Shaw on the sidelines for a match that will see the Baggies attempt to equal their Premier League club record of four consecutive home victories.
Early-season pressure on Tony Pulis has become a distant memory, with just one loss in six matches propelling the Baggies up to seventh
Mourinho has long been an admirer of the West Brom boss, saying 18 months ago that if he owned an English club he would look to make Pulis his manager.
"In England? For sure (I'd appoint him)," Mourinho said.
"I think (British managers are overlooked) because there are different kinds of profiles.
"Real Madrid, Inter, Barcelona, Bayern, Juventus, Dortmund - if you manage these big teams obviously the targets are impossible for people like Tony Pulis, Alan Pardew, Mark Hughes, David Moyes
All of them.
"And Tony got trophies everywhere in every club he got trophies.
"He was not relegated with Stoke, it's a trophy
He was not relegated with West Brom, it's another trophy
Not relegated with Palace it's another trophy
And so on, and so on.
"So he has lots of trophies
By trophies I mean reach the target and he did that
Like some, never relegated is a big trophy.
"You are not expecting Sam (Allardyce) to be champion in the clubs that he was managing.
"So for me, yes, English managers are not looked (at) as they should."
Source : PA