How to measure the gulf between the Manchester clubs? The FA Cup final provided another gauge: City's second-choice goalkeeper is more competent than United's No.1.
Goodness knows when United will decide to announce De Gea's inevitable contract renewal. There is not a good time now. Not when he has contributed to a City double and probable treble.
De Gea conceded identically for the second month running, palming Ilkay Gundogan's trickling volley into the corner of the net. He had done similarly at the opposite end of the Jubilee Line against Said Benrahma.
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United will not become a genuine force again with a goalkeeper who literally and metaphorically stands still. The City custodian Stefan Ortega had little to do with his hands but was faultless with his feet, bringing serenity to an anxious occasion. Gamesmanship, too. He was booked.
There is much United will rue from their second successive FA Cup final loss, particularly the early goals they sieved at the start of each half - the first after 12 seconds. It has been an unwanted habit of United to be so porous so soon after the referee's shrill.
That affected their game plan and they were behind the curve for the majority of a tense final. A team that has struggled for goals was struggling for goalscorers. Of the substitutes, Scott McTominay came the closest to triggering extra-time. Hardly any United players placed their silver medal around their neck.
United supporters sang "we love United, we do" as full-time loomed - acceptance of a successful season despite its downbeat denouement. The worst may be yet to come if City overcome Inter Milan in Istanbul next week.
However narrow the loss, United cannot be lenient in the transfer window. In a cup final, their squad was teeming with a surfeit of inadequate players who need to be moved on.
De Gea had required treatment in the first half and perhaps that impeded him with both goals. If so, then he should have been hooked. Jack Butland warmed up albeit without the belief he would be summoned for a belated debut.
A basic rule for a defender is not to let the ball bounce and Victor Lindelof did in the ninth second. He still connected with it before Kevin de Bruyne but the ricochet looped presentably for Gundogan. Casemiro, beaten in his aerial duel by Erling Haaland, debriefed heatedly with Lindelof. Ten Hag stood motionless.
United had conceded in the first 10 minutes of their last three defeats to City under three different managers. In each derby, City wiped the floor of them, scoring two, four and six. One City supporter wielded a prosthetic leg. It was a little early to get legless. but United were in a catatonic state.
De Gea was statuesque again as a Rodri header brushed the side of his net and the final had not reached the five-minute mark. United supporters groaned at Fred's shortcomings - a foul and a heavy touch - Jadon Sancho was flagged offside and Rodri was goal-side of his marker.
Harry Maguire and McTominay sought clarification from the fourth official over a rumbustious challenge by Ruben Dias on Marcus Rashford. That indicated desperation from the United bench.
From catatonic to containment. Fernandes slid in full-blooded on Jack Grealish whereas Sancho was too respectful of the City shirt he once donned. Not for the first time this season, the winger was fazed by the pressure of the red shirt and Sancho operated as though on borrowed time, despite a callow bench of wingers agreed 18 or 20. Sancho cannot start ahead of Garnacho again.
On the hour, Ten Hag snapped at Sancho. Alejandro Garnacho had already removed his bib and slotted his shinpads under his socks but Sancho was repreived as Christian Eriksen was withdrawn.
Both of Eriksen's stints as the No.10 this season were against City and in both games the pace was too frantic for the delicate Dane. Yet Ten Hag had little choice. United's bench featured two loanees and two wingers with two starts between them this year - in the League Cup and Europa League. Facundo Pellistri and Anthony Elanga's bibs stayed on.
Such limitations elevated Fred to certain starting status. This also has to be his final run-out in a United kit. His needless hack on De Bruyne inside United's third invited the Belgian to tee up the unattended Gundogan, loitering outside the area. Inexplicably, Fred did not see the yellow card until the 79th minute.
Ten Hag held a thorough conference in the 26th minute as the United physio attended to De Gea. Wan-Bissaka was spoken to for the third time since kick-off when he had been one of the few passable performers of the first quarter.
It was Wan-Bissaka who made United's equaliser possible, salvaging an attack from Fernandes's lofted pass, his header deviating off Jack Grealish's arm. City had the header covered - none of the United forwards were attacking the six-yard perimeter. Grealish's willingness to track Wan-Bissaka was his undoing, as was the inexplicable handball rule.
When United embarked on a breakaway - their favoured mode of attack in recent derbies - City had five blue shirts in their area to one in red. Yesteryear, United's threat would have ended there. This time, they retained the ball and worked it to the willing Wan-Bissaka.
Fernandes, scorer of a contentious leveller in the January derby, was nerveless with his pressure penalty. Lindelof, already bruised from handling Haaland, was in the wars again when a missile struck his face during the celebrations.
Guardiola, animated as ever, incurred the wrath of Ten Hag over a vain City penalty appeal during a heated exchange of opinions. The ever-voluble Grealish got involved. United's equaliser had rattled City's cage. It worked in the January derby but not in June.
With United trailing, Ten Hag was compromised. Garnacho was a predictable first entrant yet the second was Wout Weghorst, scorer of two tap-ins in 30 appearances. Some City supporters cheered his arrival. McTominay, a reasonable goal threat, was thrown on and Raphael Varane was visibly flummoxed by United's formation.
City's No.9s were Haaland and the unused Julian Alvarez. United's are Anthony Martial, their crock monsieur, and Weghorst. That is another measure of the gulf.
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