The Red Devils are 15 points clear at the top of the table, but are no longer in contention for the double after losing 1-0 to Chelsea in their quarter-final replay on Monday.
"The dressing room is disappointed," the Daily Star quotes Smalling as saying. "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.
"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 give them a good performance because the second half at Chelsea was just not good enough."
The North-West outfit will face their local rivals Manchester City next Monday evening.