“There's nothing for United to take out from this game. The players should be embarrassed," declared a clearly angry Keane
The phenomenal Erling Haaland and Phil Foden both scored hat-tricks as City thrashed rivals United, highlighting the huge gulf in class between the two sides once again.
Haaland’s treble, remarkably his third in successive home Premier League games, took his tally since joining the champions in the summer to 17 in just 11 appearances in all competitions.
Foden also had a huge hand in a rampant City display which evoked memories of their famous 6-1 derby demolition of United at Old Trafford in 2011. Foden began the rout after just eight minutes and United were a distinct second best thereafter, although
Antony pulled one back and Anthony Martial also claimed two consolation efforts.
The result, which took City back within a point of the top of the table, both underlined the champions’ brilliance and emphasised the size of the task still facing Erik ten Hag at United.
Former United captain Keane has never held back in his criticism of United when they suffer a poor defeat and he was cutting in his comments on Sky Sports.
"There's nothing for United to take out from this game. The players should be embarrassed," declared a clearly angry Keane.
"You can't go into half-time 4-0 down in a derby game. It's hugely embarrassing, as brilliant as City were United were desperate, particularly in midfield. They were over-ran in midfield and defensively they were all over the shop.
"Again we look at leadership... there was nothing there from Man Utd.
"If you're giving players like De Bruyne time and space, you're looking for trouble. United got punished and rightly so. Top level sport is fantastic but it's brutal.
"Today, it was brutal for Manchester United as when you're not quite at it, you get hammered by your local rivals.
"United have an experienced team out there. City were missing players, but whoever comes into the team is pure quality. That's what they have whereas Man Utd seemed all over the place.
"At a big club like Manchester United, you wonder what it must be like for the players sat in the dressing room 4-0 down at half-time. It must be bizarre, but then these players are getting used to it as they were 4-0 down at Brentford.
"The manager must be scratching his head with his staff as there was progress in the last four games, but this is a huge step back."
Keane suggested he "couldn't believe what he was watching", as United were torn apart by City, as he suggested a slugging opening was unforgivable in such a big game.
"It was the way they started. They were slow out of the blocks. City sent a message that they were up for it, and the first goal is huge," he continued.
"United went down after eight minutes and they never recovered. There were spaces everywhere, they were sloppy in possession. Rashford never got hold of it, the wide players gave it away.
"Every time Man City went forward, they looked like they were going to score. It's a pleasure to watch City. They're one of the best teams I've seen live but for United this is a huge setback.
"It's a huge step back. They were 4-0 down at half-time against Brentford. The performance in the second half is no good. There was no aggression in City's play by then as the game was over. They'd made loads of substitutions so it's no consolation."
Keane went on to suggest United manager Erik ten Hag is not treating Cristiano Ronaldo with respect as he left him on the bench for the whole derby.
"I think Man United are just showing disrespect to Ronaldo. I think he should have been let go before the transfer window,” he added.
"I think the manager is holding on to him - okay, you say you need options, but you don't hold on to Ronaldo to sit on the bench. He's one of the greatest players ever.
"He had options - this idea that he'd no options is rubbish, he did have options, four or five very good options.
"Okay, today the manager, he brought [Anthony] Martial on and he scored two, so yeah, that's justified, but generally speaking, the bigger picture, he's not going to play Ronaldo, we know that. He's played in one or two European games.
"It's just going to get uglier as the season goes on. Okay, Ronaldo's motivated with the World Cup coming up, but if he's sitting on the bench week in, week out, it's not good, it's just going to get ugly.
"I think United have shown nothing but disrespect to Ronaldo. They should have let him go when the opportunity came.
"As I said, he did have options to go, I know that for a fact, and holding on to him to sit on the bench I think is ridiculous for a player of his stature."