Van Persie took his tally at United to 15 goals in 21 games with the opener in Saturday's 3-1 home win over Sunderland.
"I saw the finish," said Smalling.
"It wasn't easy to put it in that top corner but he is a quality player. Obviously 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. He is putting teams on the back foot and making our job easier at the back."