The Red Devils put one foot in the final with a 2-0 triumph over the Tigers at Old Trafford, but they were forced to wait until the 56th minute to find the breakthrough following an unconvincing display in the first half.
Smalling acknowledged that his side were not good enough in the opening 45 minutes, but was happier with the second-half display as goals from Juan Mata and Marouane Fellaini gave United a cushion at the halfway stage of the tie.
"We all knew at half time that the first-half performance wasn't good enough so it was good to get the two goals in the second half," he told reporters.
"It was a frustrating night, we knew how they would set up and play but in the second half we played with more tempo and it worked for us."
United have now won nine games in a row across all competitions heading into Sunday's clash with arch rivals Liverpool.