United won the fourth round of their FA Cup tie 2-1, with Maguire heading a winner injury-time.
He said afterwards: "A great finish to the game. The second half we played a lot better. The first-half was nowhere near good enough, we played at a slow tempo and we didn't really get going. We probably deserved to go into half-time losing.
"I think the second-half we came out a lot bigger tempo, we dominated the game and it really felt like we were going to get that winner, it would fall to us and thankfully, I put it in the back of the net."
On Alejandro Garnacho's impact, Maguire added: "I think he made a big impact, he always does that when he comes off the bench, so credit to him, he's ready, he's focused. We knew at half-time he'd bring a little more direct and pace into the team."