The Belgian international striker has been an inspiration for both club and country so far this season, scoring 14 goals in 11 games for United and Belgium.
But a lack of cover at Old Trafford means that the modern phenomenon of squad rotation will not apply to Lukaku as far as Mourinho is concerned.
“Look, in here, without Zlatan we cannot rotate the striker, especially because Marcus Rashford is playing also in other positions,” said Mourinho.
“So until the moment we have Zlatan we cannot think about giving a rest to our number nine, the same way we give in other positions.
“We cannot do that. Belgium is Roberto’s (Martinez the national head coach) responsibility and it’s his decision to play him or not to play him.”
Lukaku is scheduled to join Martinez’s national squad this weekend for forthcoming World Cup qualifiers with Bosnia-Herzegovina and Cyprus.
However, Lukaku’s goals in the last round of games at the start of September saw Belgium become the first European nation to qualify for next summer’s World Cup Finals in Russia and Mourinho is clearly hopeful he may not be required to play complete matches this week.
Not that Mourinho wished to be seen to be issuing Martinez with instructions.
“It’s Roberto’s responsibility,” said Mourinho.
“I cannot, professionally speaking, say do this or do that, you know. It’s Roberto’s responsibility.”
– ‘Fighting for his evolution’ –
Should Lukaku play in both qualifiers, as well as United’s weekend meeting with Crystal Palace, it will mean he will have played 14 top-level games within the opening two months of the season — a hefty workload in the modern game, especially for an attacking player.
The only game Lukaku has not started for United was the League Cup tie with Burton, although he was even named to the substitutes’ bench for that.
Aside from the visit to Swansea in the next round of the League Cup, the only prospect of a break for Lukaku, therefore, will come when Ibrahimovic returns to action.
Mourinho has stubbornly refused to discuss the Swedish veteran’s rehabilitation from knee ligament surgery although even the most optimistic estimates put his potential return date in December.
In the meantime, Mourinho has praised Lukaku for his professionalism and his development over the years with Chelsea (Mourinho loaned him out to Everton in 2013 when he came back as manager for a second spell and then sanctioned his sale a year later), West Brom, Everton and, now, United.
“I think every step in his career he got it, he grabbed it with both hands,” said Mourinho.
“Since his first loan to West Bromwich Albion, he grabbed the opportunity with both hands, fighting for his evolution.
“He was very strong in his desire to be playing in a team with other objectives now, with Champions League football, it was always his dream to come to this level and since he arrived I don’t think he thinks about another thing other than in his success.
“So he works hard, he rests, he has a great professional life outside the football pitch.”
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