The Danish champions came from behind to win 4-3 on Wednesday night, leaving Man United on the brink of a Champions League exit.
Garnacho set up Rasmus Hojlund's second goal as the Red Devils went 2-0 ahead before Marcus Rashford was sent off.
Copenhagen then levelled things before halftime, before Bruno Fernandes put Erik ten Hag's side 3-2 up after the break.
Garnacho was spotted 'shushing' the home crowd as he and his teammates turned away in celebration.
The 19-year-old had tried to scuff up the penalty spot prior to Diogo Goncalves' penalty, which made it 2-2, according to Vavro.
read more on manchester united
Keane jokingly blames Neville for red card after infamous challenge on Haaland
Copenhagen 4-3 Man United LIVE: Roony goal has Red Devils on brink of European exit
"I saw that he tried to do something about the penalty kick, but Kevin Diks stood in front and blocked," the defender told Danish news outlet Bold.dk.
"It is the second time. And then on our track? To me, he is a clown. He has the mentality of a child when he tries something like this again. In an away game.
"[It is] one thing at home in the 97th minute, but here in the first half? I don't understand him trying to do it."
Garnacho had previously tampered with the penalty spot in the reverse fixture, which saw Andre Onana save a spot-kick right at the death.
Most read in Champions League
Manchester United told Real Madrid deceived them with deal for 'overrated' Raphael Varane
Can Manchester United still qualify for the Champions League knockout rounds?
Man United were warned by Copenhagen with giant mocking banner referencing infamous clash
I played in Liverpool team that stunned Messi and Aguero but one man knew how to stop us
United's defeat in the Danish capital, though, means they now sit at the bottom of Group A with three points and have just two games left to play.
Ten Hag's side travels to Turkey next for a difficult test against Galatasaray in a must win clash.
Jason Cundy mocks Manchester United after losing 4-3 to Copenhagen in the Champions League
Before then welcoming Harry Kane and Bayern Munich to Old Trafford, who have already qualified for the last-16.