Garnacho won both penalties as United 2-0 via successful spotkicks from Bruno Fernandes and Marcus Rashford.
Garnacho later said: "We lost the last two games it's important to be there. We want to be in the Champions League next season so I am very happy (with this win).
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"Now every game the manager said is a final. We have to win every single game, we're Manchester United. The job is done.
"The manager wants me to go one v one, to shoot, to dribble. I won two penalties. It's important.
"It's like a dream with my age. A full Old Trafford stadium chanting my name is incredible. I am very proud and happy."