Ole Gunnar Solskjaer has warned his players against complacency as Manchester United head to Southampton looking to kick on from their dream opening-day win against rivals Leeds.

Despite being without a number of key players after a disrupted summer, the Red Devils ran amok in front of a first sell-out Old Trafford crowd in 17 months.

Bruno Fernandes scored a hat-trick and Paul Pogba provided four assists in a breathless 5-1 triumph against foes Leeds that Solskjaer is desperate to build upon at St Mary's.

United ran out 9-0 victors against Southampton when they last met in February and the Norwegian wants his players to build momentum this weekend.

"Our job now is to capitalise on it, get more energy, don't get complacent," he said after a week when new signing Raphael Varane and Edinson Cavani trained.

"With regards to team news, that's given us a boost as well. Edinson's joined in, Raphael's joining in, so we're getting fitter and fitter."

Solskjaer again turned attention to the dangers of complacency when asked about Pogba's contract situation.

"Well, that's negotiations with Paul's representatives and the club," he said of the midfielder, whose deal expires next summer.

"For me and Paul we work together every day here and we keep on trying to improve, improve the team and to enjoy ourselves.

"When you win games you enjoy yourselves more and we've just got to make sure that we don't make this one game and everyone flies off on all these... you know we get big headlines and all the praise we get.

"It's about bringing more energy to the next one and being even better, so one swallow never makes a summer."

Jadon Sancho is pushing to make his first start on Sunday, when United's other marquee summer signing is hoping to be involved for the first time.

Varane was paraded around the pitch before the Leeds win, having just completed his move from Real Madrid, and Solskjaer has been mightily impressed during the defender's first week at the club.

"It's no secret that we were close or we were in there when he left to Real Madrid 10 years ago," Solskjaer said.

"Sir Alex was there speaking to them and we were close and ever since, his performances on the pitch, his demeanour, behaviour off the pitch, it tells me that that's a Man United player.

"That we could convince him to have the feeling of Man United, I think that's been in his heart ever since he went to Real Madrid as well.

"He did have a little bit of a feeling for Man United and we definitely have had feelings for him for many, many years.

"The Champions League wins he's had, the World Cup win – everything about him is class.

"He's a defender, quick, strong, good in the air, he can play with both feet, so the short answer is everything about him is what you want. He's been humble, he's been inquisitive. He asks questions.

"He doesn't want to learn by a mistake in a game, he wants to learn it on the training field before he comes on the pitch so everything about him has been impressive."

Solskjaer was coy about the part Varane and Cavani, who has returned to training after an extended summer break, could play at Southampton, but more forthcoming about Dean Henderson.

The United goalkeeper has been dealing with prolonged fatigue since contracting Covid-19 last month but is starting to make progress.

"With regards to Dean, he was tired after training so he felt that and we needed to check him up," Solskjaer said.

"Now he feels much better, he's passed all the tests that we have gone through with him and he's training.

"He's building himself up again because he's lost a last few weeks, but he's working in the gym hard, he's working out on the grass. He's not with full team training yet but he won't be too long."

Henderson's situation highlights why Solskjaer says "we need to really be careful" around Covid-19.

"They're not all double jabbed, no," the United boss said. "I am, to put it that way, and I encourage the lads to take the vaccination but we can't force anyone to do that, no.

"To my understanding we're still wanting everyone to be double jabbed and it's up to them. It's still possible to take them."