Smalling never lost belief that United could hit back, even when Andreas Weimann scored his second goal of the game five minutes after the break.
"We knew there was still plenty of time in the second half," said the defender. "All season we've been capable of getting two or three goals in each game, so we knew our chances would come. Our first goal was the important one as it didn't give us too long to dwell on it and allow them to play too deep and sit on the lead. We managed to get a goal straight away and made an impact.
"We know we can play like that from the start, I don't know why we don't, and it took that kick up the bum to get us going. I'm sure the comeback was good for the travelling fans but it did take us a while to get going."
Smalling was full of praise for Javier Hernandez and admits the striker is convinced he deserves a hat-trick.
"Chicharito made a big difference when he came on," added the centre-back. "Full credit to him. He works so hard on the training ground. He gets so many goals just coming on and he got a hat-trick today, not many people have got the ability to do that. He brought in the ball and had us all signing it so I think, to him, it's definitely a hat-trick!"