Manchester United manager Jose Mourinho says he cannot afford to give a rest to Romelu Lukaku due to a lack of alternatives in the centre-forward role.
Lukaku, 24, has made a blistering start to his United career and has been an ever present in Mourinho's starting line-up since his summer move from Everton, scoring 10 times in nine matches.
The Belgium international's only rest came during the Carabao Cup win over Burton Albion earlier this month, and Mourinho said he expected to rely on the centre-forward to lead his attack until Zlatan Ibrahimovic returns from his cruciate ligament injury.
"Without Zlatan, we cannot rotate the striker, especially because Marcus Rashford is playing also in other positions," Mourinho was quoted as saying by Sky Sports.
"So until the moment we have Zlatan, we cannot think about giving rest to our number nine the same way we give in other positions."
Lukaku joined United in a £75m ($100m) deal from Everton in July and has played every single minute of the Red Devils' six Premier League matches this season.
Mourinho said the Belgian striker was reaping the benefits of working hard to improve his game from a young age.
"Every step in his career, he grabbed it with both hands, since his first loan to West Bromwich Albion," the Portuguese stated. "He grabbed that opportunity with both hands, fighting for his evolution.
"He works hard, he rests, he has a great professional life outside the football pitch and he has great ambition.
"So it is a global evolution. It wouldn't be fair if I didn't say that every step in his career with different managers, different players and different teams is not contributing to what he is now.
"Of course now, Manchester United is a different level of expectation and he is coping well."