Paul Pogba provided a timely reminder of his enduring worth to keep Manchester United on course for Europa League glory.
Pogba, United's £89million club record signing, came off the bench and scored within three minutes to seal another famous away win for Ole Gunnar Solskjaer's side.
His United future may be uncertain, but Pogba's talent is not in doubt, as he showed with the moment of magic that ensured Solskjaer's side are in Friday's quarter-final draw.
After an insipid first-half, in which they managed just one shot on target, change was needed, with Marcus Rashford replaced by Pogba, who made his return after a six-week injury lay-off.
It proved an inspired change, Pogba scoring within three minutes of coming on, to switch the momentum in United's favour and leave Milan needing to score a goal to keep the tie alive.
But for the first 45 minutes, the Serie A side were in control, United lacking intensity and focus, despite needing to score at least once to have any chance of going through.
United did manage to get in behind the Milan defence twice early on, but that was about it for them in the first-half, the Red Devils showing little ambition thereafter.
Franck Kessie, who controversially had a fine goal ruled out for offside in the first leg, shot tamely at Henderson moments later, with Milan's first real sight threat on the United goal.
At the other end, Dan James got away from Theo Hernandez , but was dispossessed by Fikayo Tomori, the United winger sent tumbling to the turf, his appeals for a penalty in vain.
It was the turn of Hernandez to go close in the 23rd minute, the Milan left-back embarking on a surging run towards the United goal, skipper Harry Maguire getting a vital touch on the ball.
Victor Lindelof was required to produce a similarly important challenge, after Milan broke on the counter-attack, the United defender executing a superbly-timed tackle to thwart Kessie.
Milan were more assured and played with enterprise, the hosts unlucky not to score just before the break, when Rade Krunic steered his effort wide.
Bruno Fernandes, in particular, was poor, squandering possession on multiple occasions, with the rest of the United side afflicted by the same sloppiness.
But the introduction of Pogba changed the game, the midfielder making an immediate impact with the opener and jolting his team-mates out of their first-half stupor.
Milan paid the price for failing to clear the danger, with United midfielder Fred showing great desire to keep the ball alive.
The ball spilled to Pogba, who went to shoot, then executed a clever dummy which unsettled Milan keeper Gianluigi Donnarumma enough for him to scoop the ball past him and into the net.
Milan threw on former United striker Zlatan Ibrahimovic, who forced a fine save from Dean Henderson within minutes of coming on, as Solskjaer's side held out for a deserved win.
