United's defence was last breached in the league against Southampton on 30 January and so David De Gea can proudly claim not to have conceded in 537 minutes of Premier League action.
"That's linked in with all our defenders coming back now," Smalling told MUTV. "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."