MANCHESTER -- Manchester United manager Jose Mourinho has admitted Romelu Lukaku is "tired" but insists he cannot rest his star striker.

Lukaku has scored four goals in his last 19 games after starting his United career with 11 in his first 10 appearances.

The Belgium international has also been partially to blame for goals United have conceded against Manchester City and Burnley as Mourinho's team have struggled to defend set-pieces.

Lukaku has played every minute of United's 20 Premier League games so far this season and started 27 of their 30 games in all competitions.

Mourinho said the 24-year-old is running on empty but, asked whether he could afford to give the frontman a rest, he replied: "No, I can't."

He told a news conference on Friday: "First of all, people don't have to be grateful to Romelu. I have to be, not you, not the supporters, not the pundits. I have to be.

"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 your energy, positional play, you can resist, 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, was not a direct influence on just him, but he was in the picture in some of the last goals we concede, but it is 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 [criticism]."

Romelu Lukaku joined Manchester United from Everton in the summer. Tom Purslow/Man Utd via Getty Images

Mourinho used Anthony Martial, Marcus Rashford and Zlatan Ibrahimovic as his striker during their Carabao Cup campaign and started Lukaku and Ibrahimovic in tandem in the 2-2 draw with Burnley on Boxing Day.

Ahead of Southampton's visit to Old Trafford on Saturday, Mourinho insisted he could pair Lukaku and Ibrahimovic up front again this season -- but not until they are in better form.

He added: "It can happen but I think to happen they need both to be in their best moment and they are 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 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. He 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 it 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."