The Red Devils produced a hard-fought 1-0 victory over Reading on Saturday to move five wins clear of neighbours Manchester City and Smalling suggests their dominance on the competition this campaign, particularly since the start of 2013, has been made possible by a squeaky clean defensive unit.
"That's linked in with all our defenders coming back now," Smalling told the club's official website.
"It's just Phil (Jones) who is struggling but I think he will be back soon and our strength is shown by the players that we have available.
"It wasn't a pretty game (against Reading). It got pretty ugly in the second half and I think we were just happy to get the three points. That was what we needed and we got a clean sheet as well so we're happy with that.
"We knew what we had to do before City played, but (City losing 2-0 at Everton) gave us an extra incentive. We would like to have scored a few more goals but we stayed solid and got the 1-0 win."