Romelu Lukaku is suffering from the strain of his ever-present Premier League appearance record at Manchester United this season, according to manager Jose Mourinho.

Lukaku scored seven goals in as many Premier League matches in a flying start to his United career at the beginning of the current campaign.

The £75million signing from Everton has since contributed only three more in 13 subsequent appearances, while his defensive errors from set-pieces proved costly during the recent 2-1 derby defeat to Manchester City and a 2-2 home draw against Burnley last time out.

Speaking at a news conference ahead of Saturday's match with Southampton, Mourinho suggesed the physical toil undertaken by Lukaku as United's only senior striker until Zlatan Ibrahimovic's recent return from injury has gone unappreciated.

"People don't have to be grateful to Romelu – I have to be," he said. "Not you [reporters], not the supporters, not the pundits – I have to be.

"Twenty matches in the Premier League, 90 minutes. For a striker, for any player, it is absolutely incredible.

"But if you are a central defender or a holding midfielder, if you are in a position where you can control your energy and your positional play, you can resist, you can survive.

"For a striker in the Premier League to play 20 matches, 90 minutes, I have to be very grateful for his personality and his character.

"Has he been unlucky in our box in recent matches? He was in the picture – it was not just him – but he was in the picture in some of the last goals we concede.

"Is it a consequence of a player who needs a little rest or just a consequence of the moment and the occasion?

"But the guy is fantastic for me and for the team and gives absolutely everything. I have no criticism."

Mourinho was then asked whether he would look to rest Lukaku and replied, "No. I can't."

The 24-year-old began in attack alongside Ibrahimovic against Burnley but the veteran Swede was substituted at half-time, curtailing a first Premier League start since tearing his anterior cruciate knee ligament in April.

Mourinho did not rule out using the pair in tandem again but insisted he must see improvements in form and physical condition before doing so

"For it to happen, they both need to be in their best moment and they are not," he said.

"[Lukaku] is physically a monster but he is not a machine and I think he is feeling it.

"Zlatan is a man with an incredible injury and an injury that a 20-year-old young man would suffer with. Imagine a man of 36 years old and so many miles in high level football…

"It is not easy. We did that against Burnley and will do it in some more matches if we need to do.

"For both of them to play together we really need them to be at their best level."

United head into the Southampton game second in the Premier League but 15 points behind leaders Manchester City.