Substitute Fraizer Campbell headed what proved to be a consolation goal for the Black Cats with 20 minutes left on Saturday, after Stephane Sessegnon reached a ball that looked to be going out of play.
The Reds won 3-1 but have now kept only three clean sheets in the opening 17 league games, a statistic on which Smalling is keen to improve over the busy festive period.
The defender, who played the full 90 minutes alongside Rio Ferdinand and, later, Nemanja Vidic, told MUTV: “There was a bit of carelessness from us [in the second half].
“We were disappointed with the goal we conceded because we wanted to get a clean sheet. We have to make sure it doesn't cost us next time.
“Getting a clean sheet means to a lot to us as defenders but also because our record recently hasn't included a lot of them. It would have been ideal to get one today.”
Like his manager, Smalling also felt United should have scored more against Martin O’Neill’s side, though he was impressed by the Reds’ attacking prowess, and the displays of midfielders Tom Cleverley and Michael Carrick.
“It could have been more than 3-1,” he mused. “We had so many chances in both halves to make it a bit easier for ourselves, but we got the three points, which was the main thing.
“We took the momemtum forward from Sunday [against Manchester City] and Clevs and Carrick