United will end the game with 10 men as Michael Carrick picks up a knock and the lead is so comfortable, he can just have a sit down in the dugout. They've used all of their subs.

Image source, Reuters

BBC Sport pundit Robbie Savage at Old Trafford

"The difference between United and City without the ball is ridiculous. City without the ball have been really poor."

It feels a long time since Manchester City took the lead doesn't it? An awful day for the boys in blue.

Will in Kent: Yet another loss for City today. Manuel Pellegrini needs to go. I don't believe a nice guy is enough to be a good manager. The players need someone with an attitude to manage them to get the best out of them because this just isn't working. Its always so predictable attacking football with no mentality to defend.

Ethan in Rufford: What has happened to City? Awful performance.

Tom in North Wales: You have to look at that City squad when they consistently turn to a 36-year-old on the bench to get them out of trouble.

Angel Di Maria almost gets in again as Eliaquim Mangala is caught out of position. Manuel Pellegrini shields his face behind the brick surrounds of his dugout. Sacked in the morning? Sacked in the summer?

Angel Di Maria runs at the City defence and has a shot blocked, Wayne Rooney is then flagged offside as he looked to pick up the pieces. Marouane Fellaini is replaced by Radamel Falcao. Anyone can get a game these days.

Sergio Aguero has hardly touched the ball since half-time but he tries a first-time volley from a long way out - over the bar.

Alan Green
BBC Radio 5 live at Old Trafford

"A standing ovation for Juan Mata. It's good that a player of his technical ability has started to be appreciated by Louis van Gaal - he's too good not be part of things."

Mustafa Bux: Manuel Pellegerini should have been sacked after the Palace game. Woeful.

Rishi Verma: The body language of the Man City players tells its own story. Lacking in confidence and form, they're simply not interested.

Musbau Niran Famuyiw: I do seriously think Pellegrini should be allowed to see off the season and then a decision can be taken.

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4-1 up and time to bring on a £60m signing. Suddenly life is rosy for Manchester United and their fans. Angel Di Maria replaces Juan Mata.

Johnny Magrinho: 4-1? A few weeks ago Manchester United didn't look capable of scoring twice in a game let alone four times against the champions. Incredible.

Rob Williams: This may be be the type of football that gets United into Europe - but it certainly won't win Europe. Louis van Gaal still has much work left.

Fissuh H Kelelom: I never believed until today that United are really back. Anyone still in doubt?

Phil McNulty
BBC Sport chief football writer

"The Stretford End taunts Manchester City manager Manuel Pellegrini with "You're Getting Sacked In The Morning" as he stands in his technical area. This is getting very ugly for City and Pellegrini - it was only just over a year ago that David Moyes was getting the same treatment from the visiting fans here. What a turnaround."

Image source, Getty Images

It's been a long time since Old Trafford enjoyed a game as much as this.

City heading for a sixth defeat in their last eight games. Relegation form.

Image source, Reuters

BBC Sport pundit Robbie Savage at Old Trafford

"They could go and get five and six now. Look at the body language of Manchester City's players! They know they are getting annihilated. What are we seeing here from this City defence? Did Mangala really cost £32m? I think he was playing everyone on side. I think we are looking at one team going one way, and another team going another way. On this performance they are miles behind this United team."

Image source, Getty Images

Frank Lampard, who probably wishes he was taking in some cafe culture on the Lower East Side with Christine right about now, is on for City as United also make a change, bringing on Marcos Rojo for Phil Jones.

It is getting embarrassing! Chris Smalling is the latest to score as he is totally free to head in Wayne Rooney's free-kick from the left. Woeful defending. Indefensible defending.

Image source, Reuters

United playing ole stuff now, into the feet of Marouane Fellaini and out again, Ashley Young buzzing around. This could get embarrassing.

'You're getting sacked in the morning' sing the United fans towards Manuel Pellegrini.

Is he on his way out?

BBC Sport pundit Robbie Savage

"Manchester City's back four are absolutely all the over place. Defensively they have been awful. The gap between Demichelis and Mangala is massive. It's a great weight of pass from Rooney to Juan Mata and it's a great finish through Joe Hart's legs.

"A fantastic derby performance from Manchester United, who have been superior on every part of the pitch. This City team needs rebuilding - they have been battered by United today."

It's been a stale second half but that third goal for the hosts will surely kill the game off. City have been flat since the restart and now they need two goals to claim a point.