The Swede made his first start at No.10 on Boxing Day after scoring his opening goal of the campaign against Bristol City.
But he failed to link up with Lukaku, slowing play down and leaving the Belgian isolated in the No.9 role.
And Mourinho, speaking ahead of the Southampton game, noted that the pair currently weren’t in the right condition to play together.
“It can happen (again) but I think to happen they need both to be in their best moment and they are not,” he said.
“One is not because the situation in his team didn’t allow him [Lukaku] to have any rest and he’s giving absolutely everything in every minute and we just manage to give him a little rest in cup matches and even so he was on the bench for emergency situations, Champions League was about the same.
“We had to fight until the last match for qualification and the boy is tired, is physically a monster but not a machine and he’s feeling it.
“And Zlatan is a man with an incredible injury, an injury that a 20-year-old man or young man would suffer, imagine a man 36-years-old and so many miles in high level football, so is not easy, we did that against Burnley, we will do that in some more matches if we need to do but for both to play together we need them to be at their best level.”
It has already been established that Lukaku needs a rest after starting every Premier League and Champions League game so far this season, and one can only hope that Mourinho will give either Marcus Rashford or Anthony Martial a chance to impress up front instead while the Belgian spends some time on the sidelines.
The case of Zlatan, on the other hand, is a far more tenuous one. He has not brought the game power and urgency to the side so far this season, leading to many noting that his legs may have finally gone.
But we should remember that, in theory, the 36-year-old should have been out until mid-january. He deserves more time to acclimatise further to life back on the pitch after such a prolonged absence.