United have notched up five victories on the trot but have only kept one clean sheet in that time and following the 3-1 home win over Sunderland on Saturday, Smalling feels some carelessness has crept in at Old Trafford.
"There was a bit of carelessness from us [in the second half]," he told MUTV.
"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."