For the second time in four days, Ralf Rangnick's changes saved the day for Manchester United.
On Sunday, the double introduction of Anthony Elanga and Fred saw United clinch an epic 4-2 win at arch rivals Leeds, after blowing a 2-0 lead.
And it was Elanga who came to United's rescue here, saving interim boss Rangnick from a painful inquest after such an insipid performance from his team.
Elanga had only been on the pitch for four minutes when his run was spotted by Bruno Fernandes, who threaded it through to the 19-year-old, who benefited from a slip from defender Reinildo, before finding the far corner.
At Elland Road, Elanga was struck on the head by an object thrown from the crowd as United's players celebrated Fred's strike, but recovered to score the final goal.
The teenage winger showed similar mental resolve here, plundering what could prove a crucial goal in the context of the tie, after United had gone behind early on.
The 3,300 travelling United fans, packed into the highest tier of the Wanda Metropolitano Stadium behind the goal where Elanga scored, were sent into a collective delirium and hailed the club's latest young prodigy.
This may only be Elanga's first full season with United, but already he has his own chant, to the tune of Snap's 1990s hit Rhythm is a Dancer, which the travellling fans continued to sing with gusto long after the final whistle.
Given how lamentable United were up until Elanga's goal, Diego Simeone and his Atletico players will no doubt rue the fact they did not put more goals past Rangnick's side and effectively kill the tie. In the end, they paid the price for their failure to do just that.
Even though the away goals rule no longer exists, Elanga's strike shifted momentum to United, although they will have to rase their level significantly in the return leg at Old Trafford on March 15 if they are to progress.
Despite dominating possession, United did little of note with the ball, failing to register a single shot on target until Elanga's goal, which served to mask another uninspiring display.
Despite having two right-backs in Diogo Dalot and Aaron Wan-Bissaka, Rangnick decided to go with centre-back Victor Lindelof there.
It was a bold move, but United were behind after just seven minutes, the goal coming from down their right side where Lindelof was deployed.
Renan Lodi slung in a menacing cross which was met by Joao Felix, whose sublime diving header went in off David De Gea's right-hand post.
It was a deserved lead for the home side, who had United rattled early on, in particular De Gea, who looked on edge at the start against his former club.
United were second-best throughout the first 25 minutes, with Fernandes and Paul Pogba particularly wasteful with the ball, which was rarely seen by Cristiano Ronaldo, who cut a frustrated figure up front.
That frustration boiled over just after the half-hour mark, when Ronaldo was fouled by Hector Herrera and demanded he be shown a yellow card, referee Ovidiu Hategan needing to have word with United's increasingly irate No.7.
United were lucky not to concede in the final minute of the first-half, their slowness to react to a quickly-taken free-kick allowing the outstanding Lodi to cross for Sime Vrsaljko, whose header hit the underside of the bar and came back out, much to De Gea's relief.
When the half-time whistle went, United had not had a single touch in Atletico's penalty area, the damning stat summing up their lacklustre display.
Atletico began the second-half with the same intensity they brought to the first - committed, belligerent and playing with a collective ethos, everything that was lacking from United.
But their failure to add to their lead meant United were always in the game, with Elanaga having the last word and continuing his impressive breakthrough season.
