This back four will be a quiz question for years to come if United win this one. Antonio Valencia, Michael Carrick, Paddy McNair and Luke Shaw now form the last line of United's defence.

McNair is on for Marcos Rojo, who is in trouble. He needs gas and air before leaving the field on a stretcher.

Image source, EPA

The blue tide is lapping at the shores of the United defence. Martin Demichelis angers those in red with a full-blooded tackle, Marcos Rojo retaliates with one of his own and looks like he's popped a shoulder out.

Paddy McNair - you're in son...

Image source, PA

Jon:, external Disappointed. Thought we had slim chance of a win today, Chris Smalling ruins it. Avoiding embarrassment number one priority. Title no chance.

Cairan Ryan:, external Sad to see United's collective brain cell of a back line prominently on display today. David De Gea the man of the match so far.

Sultan:, external What does van Persie mean in Dutch? Anonymous?

Image source, Getty Images

A sending off, and one which was enforced by a police officer! That's what happened to Manchester United midfielder Lou Macari at Maine Road in 1974.

Macari's on-field clash with Manchester City hardman Mike Doyle saw both players shown red. With both men refusing to leave the pitch, the referee, Clive Thomas, took both teams from the field and escorted two policemen into the dressing rooms to ensure Macari and Doyle did not return to the action.

The match ended 0-0.

Read more in our A-Z of the Manchester derby.

The siege is well and truly on now. Send for the trebuchet!

City have two penalty appeals turned away as first Michael Carrick and then Marouane Fellaini block shots at close quarters, before Pablo Zabaleta slams a shot over on the stretch.

City have Edin Dzeko, Samir Nasri and, er, Scott Sinclair available on the bench too...

Image source, Getty Images

Nicolaas Wellens:, external Hairdryer in Dutch is: 'Haardroger!' (see 14:31)

Michael Carrick clears his lines with a great header - the emergency centre-half may have to put in the defensive performance of his life in the next 40 minutes.

I remember Emile Heskey having a stormer as an emergency defender when Wigan stayed up once...

MOTD's Guy Mowbray at Etihad Stadium

"If City fail to win this now with 45 minutes to go against 10 men then their confidence might take a bit of battering."

Back under way at the Etihad. A siege from the boys in blue?

Image source, Opta

Wayne Rooney is playing very deep and rarely strayed from the centre circle in the first half. Chris Smalling's sending off is unlikely to see him further up the pitch in the second half either.

Wayne Rooney leads his nine team-mates back out onto the field, a good few minutes early. What's Dutch for Hairdryer?

Image source, Getty Images

Paul Lummis:, external Surely somebody else can see the fact that Marcos Rojo got a toe to the ball before contact with Yaya Toure? (see 45 mins)

David Broome:, external Looked a penalty to me. United have no defenders worthy of the name except Shaw. Rojo, Smalling, Jones, Rafael all pea brains.

Ross Gordon:, external Two penalties that should have been given and United should be down to nine. Very lucky to get in level but City to go on and win.

If Marcos Rojo had conceded that stoppage-time penalty for his challenge on Yaya Toure, he would surely have seen red too. He was the last man six yards out.

He came through the back of Toure but does appear to get a toe on the ball. Could go either way - but referee Michael Oliver was not in a good position, screened by at least two players.

Former Manchester City winger Trevor Sinclair
BBC Radio 5 live at Old Trafford

"It's all about creativity from here. It could be a good time to introduce a player like Samir Nasri. He has an eye to lay people off and play people in. If Manchester City can move the ball with the same urgency that they have in that first half then they will win this game.

"I thought Manchester United could have had a second sending off for Marcos Rojo's challenge on Yaya Toure (see 45 mins). It should have been a penalty - Rojo wildly flung his legs around him and denied the goalscoring opportunity."

Chris Smalling's red card was the earliest red card in a Premier League Manchester derby.

Smalling is the 26th United player to be sent off in the Premier League.

Image source, AP

MOTD's Martin Fisher at Villa Park

"There's a touch of deja vu about Aston Villa's recent slide, which is all too reminiscent of performances at the tail end of last season.

"Paul Lambert's side may have got out of the blocks quickly in August and victory at Liverpool had them sitting second just seven weeks ago, but few Villa fans were fooled.

"While defeats against Chelsea, Arsenal, Manchester City and maybe even Everton were expected, Monday's loss at struggling QPR was less acceptable and a sign of the size of their problems.

"No team in the country has scored fewer goals than Aston Villa's paltry total of four."

As we chew over the events at the Etihad a reminder that there's a game to come from Birmingham...

BBC Sport chief football writer Phil McNulty at Etihad Stadium

"Plenty of talking points at half-time - but no debate surrounding Chris Smalling's brainless sending off. His recklessness in getting a red card has left Manchester United facing a long, hard second half.

"Referee Michael Oliver has had Wayne Rooney in his ear, Joe Hart in his face and Manchester City's fans in uproar as they believe they should have had at least one penalty and possibly two.

"City dominated after a quiet start and United must now play with a reshuffled central defence after the departure of Smalling."

MOTD's Guy Mowbray at Etihad Stadium

"Now then master tactician (Louis van Gaal), what have you got? If Manchester United do manage to win, it will be one of the more famous Manchester derby wins."

Should Manchester City have had a penalty right at the end of the half? They thought so, furious as the players left the field.