The Manchester-born United academy graduate, who joined the club at the age of eight, left in a £16 million ($24.2 million, 22.3 million euros) deal last September after being deemed surplus to requirements by new manager Louis van Gaal.
And he returned to haunt his former club as Arsenal claimed a first victory at Old Trafford since 2006.
Welbeck pounced on a mistake by Antonio Valencia to score a 61st-minute winner after Wayne Rooney had cancelled out Nacho Monreal’s opener, and United’s evening ended in ignominy when Angel di Maria was sent off after receiving a second yellow card for manhandling referee Michael Oliver.
United’s wait for FA Cup success will now stretch into a 12th year and their sole objective is now the race for Champions League qualification, in which they currently trail Arsenal by a point.
With Jonny Evans beginning a six-game ban for spitting at Newcastle United’s Papiss Cisse, Luke Shaw came into the United defence, but although that was the only change to the hosts’ line-up, they displayed uncertainty from the off.
Van Gaal again played Marouane Fellaini in support of Rooney and the Belgian illustrated his unique strengths by chesting down a flighted pass from Di Maria and teeing up Ashley Young, who shot over.
Alex Oxlade-Chamberlain started on the right for Arsenal, with Mesut Ozil in the centre, and in the 26th minute the England midfielder marked his return to the starting XI by creating the opening goal.
Having earlier been granted the space to put a cross-shot wide, Oxlade-Chamberlain wriggled between Daley Blind, Shaw and Valencia before stabbing a pass to Monreal, who beat David de Gea at his near post.
– Delight –
United’s fans were quick to voice their annoyance, but they were off their feet just three minutes later when Rooney met Di Maria’s right-wing cross with a fine diving header to claim his 14th goal against Arsenal.
Encouraged, United finished the first half strongly, with visiting goalkeeper Wojciech Szczesny saving from Rooney, Di Maria and Fellaini, although Santi Cazorla worked De Gea with a free-kick at the other end.
United made two changes at half-time, introducing Phil Jones and Michael Carrick for Shaw and Ander Herrera, while Oxlade-Chamberlain had to hobble off five minutes in after appearing to injure his hamstring, with Aaron Ramsey coming on.
There were half-chances for both teams, De Gea saving a deflected Alexis Sanchez shot and Di Maria shooting wide, before Valencia’s blunder allowed Welbeck to put Arsenal ahead.
United’s supporters have bemoaned their team’s predilection for back-passes in recent weeks and it was from such an action that the goal stemmed, with Welbeck seizing on Valencia’s under-cooked pass, touching the ball past the onrushing De Gea, and rolling home.
There was no muted celebration either, with the former Stretford End favourite clenching his fists in a clear show of delight.
Smalling should have equalised when he blazed Carrick’s flick over the bar from six yards, while Fellaini tested Szczesny with a header, but it took a stunning De Gea save to prevent Cazorla swelling Arsenal’s lead.
Di Maria saw red with 13 minutes to play when he was booked for diving and responded by tugging Oliver’s shirt, completing a miserable night for United.