The eagerly awaited meeting between Pep Guardiola and Jose Mourinho did little to settle the contest for fourth place and Champions League football but, in Fellaini’s rush of blood after 84 minutes, it provided one of the season’s most memorable flashpoints.
Having been cautioned for a foul on Aguero, within six seconds of the restart United’s midfielder again tripped the City man from behind and might have escaped without a second yellow had he not followed that with a headbutt on his victim.
It meant a furious rearguard action for United and, in injury-time, City substitute Gabriel Jesus had the ball in the net only for his effort to be ruled out for offside as Mourinho saw his team extend their unbeaten league run to 24 games.
United captain Michael Carrick said: “It’s a point gained for us. We thought we could catch City on the break and we did have a couple of chances but overall I thought we did well except at the end when there was a bit of excitement. It makes things more difficult when you go down to 10 men.”
Before a ball had even been kicked, Mourinho sprung a major tactical surprise by pressing midfielder Henrikh Mkhitaryan into duty as United’s lone striker.
It was a decision that relegated Wayne Rooney to the bench but which, more importantly, allowed United to field the pacey youngsters Anthony Martial and Marcus Rashford on the flanks, from where Mourinho obviously expected them to cause City problems.
It was a tactic that almost paid dividends in two minutes as Rashford sped away from Aleksandar Kolarov and laid the ball back for Ander Herrera whose dangerous curling cross forced Pablo Zabaleta into conceding a corner.
But City soon settled into the rhythm of a derby that had taken on such huge significance for City and, in particular, Guardiola.
Already resigned to ending the first of his nine years as a manager without a trophy, defeat to the Reds and his old adversary would have left him in real danger of also missing out on the Champions League — hardly a possibility the club’s billionaire owners could have envisaged when they appointed him last summer.
Aguero, so impressive with a dozen goals in his last 12 games, might have helped ease City nerves on nine minutes when good work on the right from Kevin de Bruyne saw the Belgian cross to the corner of the six-yard box for an Aguero shot which hit the foot of the United post.
And just after the quarter-hour mark, a De Bruyne free-kick flew tantalisingly across, and through, the visitors’ area without players from either side able to make contact.
By then, Rashford and Mkhitaryan swapped positions, and the temperature of the contest had ratcheted up.
When Nicolas Otamendi appeared to step across Rashford, body-checking him and preventing him surging upfield, referee Martin Atkinson waved play on, Mourinho reached hysterically on the sidelines, and the vast majority of the Etihad crowd vented in his direction.
It was shaping into a “proper” derby, the tenacity of the tackling absorbing and United, in particular, required to defend doggedly and with discipline.
De Bruyne had a momentary space and curled a shot at David de Gea just before the quarter-mark and Eric Bailly was forced into a magnificently timed tackle to stop the progress of the increasingly influential Leroy Sane.
City’s fragility at the back, and in goal, has been apparent all season and after a couple of anxious moments for Otamendi, isolated against Rashford, those problems were almost fully exposed on 24 minutes. Martial raced away from Vincent Kompany on the left and crossed where Claudio Bravo flapped embarrassingly at the ball, laying it into the path of Mkhitaryan who struck his shot directly at the prone goalkeeper.
A rare Herrera mistake saw the ball with Kolarov, whose cross just missed Aguero at the far post. The Argentinian maintained the City pressure; first producing a curling shot which de Gea held well and then cleverly making room for himself just inside the area and shooting over.
Kolarov also came close to breaking the deadlock, from a short De Bruyne pass, unleashing a fierce low drive which de Gea kept out well at the foot of his post.
A minor confrontation between Herrera and Fernandinho saw Atkinson lecturing players from both sides and, after a clever one-two with Aguero, Raheem Sterling should have done better than advance into space and shoot high over the goal.
For all the City domination, however, they might have gone in at the interval a goal down as Rashford’s far-post free-kick picked out the run of the unmarked Herrera whose header was poor and off-target.
City maintained control after the restart — Sterling ended an electrifying run with a weak shot, De Bruyne’s cross just eluded Fernandinho, and Aguero again showed great footwork in the area but his shot lacked power.
De Bruyne cut in, with intent, from the left and saw a strong shot deflected behind and Aguero should have done more than clear the bar following a poor Antonio Valencia clearance.
Otamendi’s set-piece header deflected over, via Bailly, while Sterling’s shot, was telegraphed directly at de Gea.
Bizarrely, on a rare moment in which Bravo was called into action to catch a corner, the City keeper landed awkwardly and injured his calf, requiring Guardiola to replace him with Willy Caballero for the final 12 minutes.
Bravo 5 (Caballero 78, 6); Zabaleta 6, Kompany 7, Otamendi 6, Kolarov 6; Fernandinho 8; Sterling 6 (Jesus 86, 6), Toure 7, De Bruyne 7, Sane 7 (Navas 79, 6); Aguero 7. Subs (not used) Sagna, Fernando, Clichy, Garcia.
De Gea 7; Valencia 6, Bailly 9, Blind 8, Darmian 6; Herrera 5, Carrick 6; Rashford 6 (Young 90), Fellaini 4, Martial 5 (Lingard 79, 6); Mkhitaryan 5 (Fosu-Mensah 85, 6). Subs (not used) Romero, Rooney, Shaw, Tuanzebe. Referee: M Atkinson 6