Van Persie took his tally at United to 15 goals in 21 games with the opener in the 3-1 home win over Sunderland.
That puts him on a par with the legendary Ruud van Nistelrooy's introduction and means the 29-year-old has now scored an incredible 60 times in his last 72 Premier League games.
For Smalling, the achievement is even more remarkable given Van Persie had barely played a pre-season game before he was pitched into his debut at Everton in August, 24 hours after completing his £24million move from Arsenal.
Smalling said: "He only came in the summer and didn't have much of a pre-season. But he came straight into the matches and has been such a threat."
Even Sir Alex Ferguson must be slightly taken aback at how well Van Persie has settled into the Old Trafford fold. He said: "You are seeing a player who has come to his mature years."
Van Persie's latest effort was followed by Tom Cleverley's first Premier League goal at Old Trafford and Wayne Rooney's fifth in four games.
Yet, other than the victory that preserved United's six-point lead at the top of the Barclays Premier League, perhaps the most significant event was the return of Nemanja Vidic.
Plagued by injury for 16 months, during which time he has made only 13 appearances, the Serbian provides the most obvious answer to United's continuing defensive ills, despite being partially responsible for Frazier Campbell's goal.
Across the city, Gael Clichy has told his Man City team-mates to roll up their sleeves for a fight. The France international helped City to a battling 3-1 Barclays Premier League victory at Newcastle to ensure they maintained pace with arch-rivals United.
He said: "Last season was a great season, but like the manager says, we have to put this behind us and start to work."