Roy Keane was left dumbfounded by Manchester United's first-half performance against Man City, before insisting that the occasion was 'too big' for the visitors after Erik ten Hag's men conceded four times in a disastrous opening 45 minutes.

Phil Foden opened the scoring early on for Pep Guardiola's side before a quick-fire brace from Erling Haaland put City into cruise control.

Foden then got his second thanks to Haaland to cap off a brilliant first half for City, resulting in hoards of United supporters heading for the exit.

Roy Keane has slammed Manchester United's first-half performance in the Manchester derby

And Keane said he 'couldn't believe' what he was seeing at the Etihad.

'I'm shocked with how poor United have been, City have been excellent,' Keane said at half-time. 'You give Man City this time and space with their quality, it's far too easy. No aggression, try and force them back. Worst start ever for United.'

Keane slated United's performance and said Ten Hag's men were looking disjointed all over the pitch. 

'Maybe a lack of belief,' Keane said when asked about United's poor performance. 'It’s been easy for City. As poor as United have been defensively, their passing’s been sloppy, there are gaps between McTominay and Eriksen. 

'Huge job for the manager at half-time. I cant believe how bad Manchester United have been. 

'United are all over the place but it’s brilliant for City. Manchester United, oh my God, it’s like the game is too big for them. 

'The occasion has got to the United players. I almost can’t believe what I’m watching.'

Haaland and Foden both went on to seal their hat-tricks, while Anthony Martial's brace and a Antony superb strike ended up just as consolation goals as the nine-goal thriller ended in City's favour.

And Keane insisted at full-time that United's players should be 'embarrassed' of how they performed.

'It was the way they started. Obviously the first goal is huge, goes in after eight minutes and they never recovered,' Keane summarised. 'Every time City went forward, they looked like they were going to score. 

'Watching Man City live, it’s a pleasure. They are one of the best teams I've ever seen. But Man United, it’s a huge setback for them.

'The performance in the second-half, it’s not good. Even the consolations, they scored a few goals but the game’s over, Man City had switched off. 

'From the players point of view, they really should be embarrassed. You can’t go in at half-time in a derby game 4-0 down, it’s hugely embarrassing. As brilliant as City were, United were desperate. Particularly in midfield. Defensively all over the shop.'

Haaland is now a history-maker after scoring his third successive hat-trick at home in the Premier League, continuing his unrivalled and blistering start to his debut campaign at City.

Guardiola's men move to within a point of leaders Arsenal after tearing apart their bitter rivals.