At around 2.09pm yesterday Chris Smalling was led by officials into an empty dressing room inside the Etihad Stadium.

The Manchester United defender would probably have preferred that he be conducted into a spare janitor's cupboard, anything with a lock on the door. He had just been sent off in the city derby, his team playing away from home and half-time just minutes away.

The whistle acted like a starter's pistol releasing Louis van Gaal from his technical area, the United manager making his way down the tunnel immediately to confront his errant player. Their conversation was missed by the various microphones dotted around the stadium but it would not remain entirely private; Van Gaal gave journalists the gist later on.

He referred to Smalling as "stupid" for incurring a second yellow card - the defender was booked first for blocking a kick-out and again for clattering James Milner - reiterating that his actions were "not very smart".

The harsh words spoken within a dressing room are often only ever whispered outside its walls but, if there is a rule book which states that criticism between colleagues must remain private, then Van Gaal picked up that tome and duly smacked his defender round the back of the head with it.

The United manager was in little mood to protect his player since the dismissal left a hole in his side's defence, a gap which midfielder Michael Carrick was sent on to try to fill. United's backline had been wounded and City began to circle, with Sergio Aguero and Yaya Toure both probing hungrily. The latter should have been awarded a penalty moments before the break when he was brought down by Marcos Rojo.

Had referee Michael Oliver given the foul then the Argentinian would have been sent off too, but instead he was ushered from the field only after dislocating a shoulder. "We have many injuries in defence now, and a player suspended," said Van Gaal.

Weakened at the back, the Old Trafford side were unable to fend off their hosts for more than an hour. It was a goal rendered brutal by its precision: Toure clipped the ball wide for Gael Clichy, whose low cross was skelped into the net by Aguero. That was it, City were 1-0 up.

Manuel Pellegrini's side will have been determined to produce an uplifting result from their first derby of the campaign since Chelsea continue to float some six points above them in the Barclays Premier League table. There was one unpalatable moment for the home side - Marouane Fellaini was later accused of spitting on Aguero after the striker went down in the box, although the gobbing did not appear intentional - but Pellegrini remained satisfied with the efforts of his team.

"It is always important to win a derby, especially against this United team which is a great team with great players," said the City manager.