United came from behind to win 2-1 at Old Trafford.
"We are very happy. But I think we also played a good performance in the first half - we came in (at half-time) 1-0 down. But I think we played, in general, a very good game," Garnacho told MUTV afterwards.
"I missed two big chances in the first half, but I came back in the second half with the same mentality, trying to score and win the game, and, finally, I could do it.
"I know (Marcus Rashford can do this) because we tried it in training this week. I know he has a very good cross and I have to be ready in the back post. When he crossed it, I shot first time and I scored, so I’m happy."
On fellow goalscorer Rasmus Hojlund, Garnacho added: "I think the goal was the worst part of his performances because he held the ball very well and gave breath to the team! It was important for us and for the counter attack, so I'm very happy for him also."