The 23-year-old was a part of the side that lost the quarter-final replay to Chelsea, but with a huge game up next at Old Trafford, he says they have to get over the defeat quickly
Smalling played at Stamford Bridge as United crashed out to Demba Ba's goal in the second half, so ending their hopes of a league and cup double this season.
But while the 23-year-old admits the defeat was hard to take, he is adamant United need to repay their fans with a good derby performance against City.
"It will take a couple of days to get over this because a lot of us came into it with great confidence and we felt we were going to win this game," Smalling told MUTV.
"At least we have a good game next Monday but we need to make sure that we rally together and win this title with aplomb.
"Our fans will be right behind us and we need to make sure that we give them a good performance because the second half today was just not good enough.
"The dressing room is disappointed. I think the first half went very well and we were on top, playing some good football so a lot of us are disappointed that we didn't carry that on into the second half, which wasn't like us.
"We were very confident that we would score today and judging by our first-half performance, we thought we wouldn't concede either because they didn't create too many chances - but it was really unlike us not to score.
"In the last half an hour, the game started to stretch which wasn't ideal for us but before that we felt comfortable as a team and played some good football in the first half.
"The biggest disappointment is that we didn't carry on in the second half. That's what everyone is down about.
"At the start of the day, we saw this replay as a new challenge and a fresh start so you can't thinkg about the lead in the first match and what might have been.
"We need to make sure that we remember this defeat next season and not let it slip again."