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 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 Ibra out, Mourinho said: “It is, it is.
“But Saturday is the last effort and then they will have a week without football and that week is a week 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 the St Stephen’s Day clash with Sunderland will be welcome respite for a player Mourinho will handle with care 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.”
That rest won’t come tonight, though, and Ibrahimovic could be joined in the starting line-up at West Brom by Chris Smalling after the England defender recovered from a foot complaint. Smalling’s return is a welcome boost to a defence without Eric Bailly due to a knee injury.
Bailly will join Henrikh Mkhitaryan and Luke Shaw on the sidelines.
Mourinho is not “chasing” any January signings but Old Trafford departures could force him into the market. Speculation is mounting with Morgan Schneiderlin, Memphis Depay and Bastian Schweinsteiger among those linked with a move away.
Mourinho is again reportedly eyeing a move for Southampton captain Jose Fonte, but the United boss is not expecting a frantic month of transfer activity.
“No, I am quiet and calm and see what happens,” he said when asked about incomings.
Meanwhile West Brom boss Tony Pulis is braced for another Saido Berahino transfer saga - despite the striker’s lack of action. The 23-year-old has not played since September but is out of contract end of the season.
Albion, aiming to equal their Premier League club record of four consecutive home victories, are hopeful he will sign a new deal, and are in no rush to sell the former England U21 international for a cut-price fee. Tottenham had four bids rejected in 2015, leading Berahino to threaten to strike, while Stoke and Crystal Palace offered around £20million in the summer.
Pulis expects more transfer window drama with the striker in January.
“It’s happened before and it’ll happen again, there will be people talking to him and telling him what he should and shouldn’t be doing. It’s up to him to work it out,” Pulis said.
“The window opens again and you don’t know what’s going to happen.”
Berahino will not feature against Manchester United today as he regains fitness, having been ruled unfit by the club and sent to France on a training programme.