Juan proved there is life in the old dog yet - as a young pup made his bones for Manchester United.
Mata created one goal for Scott McTominay and then scored one himself before Paul Pogba came off the bench to smash home a deflected free kick.
But United were indebted to Dean Henderson, who made a stunning one-handed save from Leandro Trossard with the score at 1-0.
The United keeper, 23, is back at the club after impressing on loan at Sheffield United and he did not have a lot to do in this game.
But he made the big save when it mattered to show he has what it takes to put pressure on David De Gea.
And it was 32-year-old forgotten man Mata who dragged United through, rolling back the years to give a masterful display.
Mata is yet to make the squad for a Premier League game this season but has started twice in the Carabao.
And boy has he made an impact, scoring against Luton in the last round before running the show here.
Mata only started eight league games last season as well but he contributed two goals and five assists in the Europa League as United reached the semi finals.
And he has actually now been directly involved in 13 goals in his last 15 starts across all competitions.
This game had a lot to do to get close to the levels of the drama between these two teams at the weekend.
But the two teams were almost unrecognisable with both managers ringing the changes and the Premier League clearly the priority.
Brighton tore United apart on Saturday, hitting the woodwork five times and still somehow managing to lose, with a little help from VAR.
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This time the were quickly put under pressure by the visitors, and Odion Ighalo should have put them in front.
Played in by Mata, his touch took him too far wide and his off-balance shot skimmed the outside of the post.
Every time Mata was on the ball, United went up a gear. But Henderson got down well to parry a shot from just outside the box by Alireza Jahanbakhsh.
And when Bernardo was released down the left and raced in on goal it looked for all the world like Brighton might grab the lead, but Eric Bailly made a fine recovery tackle.
But just before half time Mata won a free kick in a dangerous position and floated in a pinpoint cross for McTominay to head home.
Fred had a free kick saved at the start of the second half, and it wasn’t long before Graham Potter sent on big guns Neal Maupay and Leandro Trossard.
The two almost combined to fashion an equaliser when Maupay fed Trossard and he hit a shot which Henderson somehow turned aside from close range.
It was a superb save at a vital time, underlining just why Ole Gunnar Solskjaer wanted him back at Old Trafford this season.
And it was only when Mata danced through a static Brighton back four to slot home coolly through Lewis Dunk’s legs past Jason Steele that United had the game won.
When Pogba stepped off the bench to smack home his free kick, it was curtains for Brighton, and the quarter finals for United.
Brighton (3-4-3):Steele 6; White 6, Dunk 6, Burn 6; Veltman 6, Molumby 6 (Sanders 81), Gross 6, Bernardo 6; MacAllister 6, Gyokeres 5 (Trossard 63, 7), Jahanbakhsh 6 (Maupay 51, 6).
Subs: Sanchez, Maupay, Trossard, Cochrane, Roberts, Jenks
Manchester United (4-3-3):Henderson 7; Dalot 5, Lindelof 6, Bailly 7, Williams 5; McTominay 7, Van de Beek 6, Fred 6 (Lingard 81); Mata 8, Ighalo 5 (Rashford 69, 6), James 5 (Pogba 69, 7). Subs: Greenwood, Grant, Lingard, Fosu-Mensah, Mengi
Referee: Graham Scott 6