Mainoo could have won the game for United, but a late penalty gave Liverpool parity in a 2-2 draw.
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Speaking to Sky Sports, the England international said: "I mean, of course, it's an unbelievable feeling; first professional Old Trafford goal. And to do it in this fixture is also amazing.
"But, as I say, at the end of the day, we don't walk away with the three points, so I can't be too happy."
"I feel like it's just little mistakes," he added.
"On the ball, off the ball, you know? I mean, they happen in games, but sometimes they can just be detrimental, and I feel like that's what's cost us."