Jose Mourinho said Romelu Lukaku's form has suffered because he is exhausted – but claimed he cannot afford to rest him.
Lukaku, a £75million summer signing from Everton, has scored just four in his last 19 games in all competitions, having started the season with 11 in 10.
Manchester United boss Mourinho admitted the forward is tired, but claimed he is unable to rest him because he does not have alternative options up front.
Zlatan Ibrahimovic is still way short of match fitness and sharpness after a seven-month injury lay-off, while Mourinho does not trust Marcus Rashford and Anthony Martial there.
Asked if he was able to rest Lukaku, Mourinho said: “No I can't. The boy is tired. He's physically a monster, but not a machine, and he's feeling it.
“First of all, people doesn't have to be grateful to Romelu, I have to be, not you, not the supporters, not the pundits.
“I have to be, and the other day I said 19 matches in the Premier League, now I say 20 matches in the Premier League, playing 90 minutes, I think for a striker, any player, he's absolutely incredible.
“But if you are a central defender, a holding midfield player, where you can control you're energy, positional play, you can resist and survive.
“But for a striker to play 20 matches in the Premier League, 90 minutes, I have to be grateful. Is he unlucky in our box in recent matches?
“He was in the picture. It was not a direct influence on just him, but he was in the picture in some of the last goals we conceded, but it's a consequence of a player that really needs a little rest.
“But the guy is fantastic for me and for the team and gives absolutely everything and I've no critics.”
Mourinho started Lukaku and Ibrahimovic together for the first time on Boxing Day in United's 2-2 draw at home to Burnley.
But Ibrahimovic was hauled off at half-time after an ineffective display which showed why he cannot be used to give Lukaku a much-needed rest.
Asked if they can play together successfully in the future, Mourinho said: “It can happen but I think to happen they need both to be in their best moment and they're not.
“One is not, because the situation in his team didn't allow him 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 then he was on the bench for emergency situations.
“The Champions League was about the same. We had to fight until the last match for qualification.
“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 it's 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.”