The Premier League honeymoon for big Harry Souttar has been rudely interrupted by a nightmarish afternoon at Old Trafford's Theatre of Dreams.

The only Australian playing in England's top flight had begun life for his new club Leicester with two wins and plentiful paeans of praise for his stoic displays in the victories over Aston Villa and Tottenham.

But the reality check was jolting on Sunday as Manchester United's attack, led by a rampant Marcus Rashford, ripped through Leicester's shaky rearguard for a 3-0 win which exposed just how much work the Socceroo still has to do in cementing his fledgling centre-back partnership with Wout Faes.

In fairness to Souttar, it was the performance of his wayward Belgian partner at United's home which most exposed the Foxes on an afternoon when they had begun with real promise.

Souttar had only previously ever played one match at Old Trafford in an under-23 game and explained how his granny and other members of his Scottish family had all trekked south to enjoy his big afternoon.

At first, it all looked as if they'd have a great trip as Leicester dominated the early stages and could have gone two up if not for terrific saves from David de Gea.

Indeed, Souttar, who recorded his first assist in setting up one of the four strikes against Spurs the previous weekend, even threatened his own maiden top-flight goal but headed powerfully over from a ninth-minute corner.

With a couple of clearing headers and a coolness to play out from the back under pressure, all was going swimmingly until Faes' poor ball out from the back was snaffled by United, who struck instantly through Bruno Fernandes' excellent ball in behind to Rashford.

Souttar was caught napping by his teammate's error, not reacting quickly enough and playing Rashford onside. The striker needed no invitation to bury his shot in the corner.

That goal and the failure of the referee to send off Marcel Sabitzer for a dreadful knee-high challenge on Faes changed the whole complexion of the game, with the centre back pairing never really looking on the same wavelength.

Gaps increasingly opened up in the second half, with Rashford terrorising the backpedalling Australian international and Faes getting caught out by playing United's England striker onside as he raced in to make it 2-0 with his 24th goal of the season in all competitions.

Souttar's scream of frustration as United again got in behind to score a third through Jadon Sancho told its own story of the Socceroos' miserable afternoon.

"You just have to defend better against quality players," Leicester boss Brendan Rodgers said of his backline.

But he remained upbeat, suggesting: "I have seen enough over the last few weeks to say we are getting back on track."