The Red Devils snatched the three points through a late winner from new signing Joshua Zirkee at Old Trafford.
However, they should have made it 2-0 in injury time when Marcus Rashfordcapitalised on a defensive error to run clean through on goal.
The Englishman was unselfish, squaring the ball to Garnacho who somehow managed to turn it wide with an empty net in front of him.
And fans couldn't believe their eyes with one posting on X: 'like HOW IS THAT POSSIBLE'.
Another made light of the moment, writing 'even grandpa would have scored that one'.
Others claimed Rashford was partly to blame for playing the ball slightly behind the Argentine with one saying the pass 'wasn't great'.
One viewer summed it up by posting: 'the Bassey clearance, the Rashford pass, the Garnacho finish. Maybe the worst sequence of play this sport has ever witnessed.'
It was Garnacho who most of the focus was on, however, and one man made sure to go out of his way to console him after the final whistle.
Assistant manager Ruud van Nistelrooy made a beeline for the 20-year-old to put his arm round him and tell him to keep his head up.
Speaking after the game, manager Erik ten Hag was critical of his team's wastefulness.
'I think one area we have to get right is how to kill (games) in the box,' Ten Hag told reporters. 'We saw it in the friendlies and last week against (Manchester) City (in United's Community Shield loss).
'We created enough chances and should not to have wait for a late winner. We have to be more composed and kill teams.
'Maybe it is because it is the start of the season, players have to come into full speed. We have enough players with scoring abilities.'