Brown, a seventh-minute substitute for Ronny Johnsen, came on just in time to celebrate Ruud van Nistelrooy slamming United's opener past Francisco Molina after Ryan Giggs had forced his way into the area.
Brown settled quickly into the laconic style of repelling strikers which is the essence of Laurent Blanc and Barthez. In the 37th minute, though, Blanc and Brown were made to pay for an overly casual approach when Diego Tristan was able to feed Sergio in behind them.
The goal was at Sergio's mercy but Brown appeared to have recovered well with a timely foot that should have allowed his goalkeeper to clear the danger. Hesitation turned to farce when the two collided, allowing Sergio to tap home the equaliser.
Even before United's red faces had returned to a normal colour, Deportivo humiliated their hosts again. Victor swung a cross to the far post and Diego Tristan was one of two Deportivo players going up against one United defender.
Gary Neville, who should have been there, was missing and the unchallenged Diego Tristan looped his header around Barthez and into the net. United were stunned and the crowd fell into a silence not experienced at Old Trafford since Real Madrid routed them here in the quarterfinal of the competition two seasons ago.
Ferguson was so affected by that defeat that when United set out in last season's competition, he vowed that his team would rarely play 'like United are supposed to play' in Europe again.
One year on and they are still struggling to find the right balance-between trying to control every situation and battering opponents senseless with continuous attacks.
There was little time for introspection last night and within two minutes of United having gone behind, Neville atoned for his error with a pass which deflected off David Beckham and found Van Nistelrooy. Striding through two defenders, the Dutchman appeared to have gone too wide before threading his shot precisely into the roof of the net.
That passage of play was nothing short of sensational - the kind of non-stop action which can only be the product of superb strikers pitted against very bad defending.
Deportivo were slack at the back and United resumed in the second half in much the same lackadaisical manner in which they had ambled through the first. Diego Tristan ran amok on the left and crossed for Valeron but this time Barthez was in more familiar form and parried the drive wide.
Valeron opted to go it alone next time but screwed his effort wide before a hopeful Deportivo clearance to the edge of the United area on the hour induced a reprise of Barthez and Brown's version of musical chairs.
Brown had the ball covered but Barthez ran from his line to clear from the onrushing Diego Tristan only to go to ground just as he should have kicked the ball to safety. Brown stood as if paralysed while the Deportivo striker skipped around them both to slot home the winner.
Both Andy Cole and Van Nistelrooy could have equalised in the last five minutes if they had hit the target but despite the valiant efforts of the Dutchman in particular, a draw would have been unfair on Deportivo.
Having overcome their travel sickness in Europe by winning in Athens last wednesday, no-one could have expected United to lose at home in Continental competition for only the sixth time in their history a week later.
The sale of Jaap Stam has done little to improve their shortcomings at the back and Johnsen's hamstring injury will rule him out for the next five weeks.
But the damage done to the team's confidence in Europe could yet prove the worst legacy of a bad night for United.
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