The Dutchman, who arrived from Arsenal in the summer, netted United's opener in the 16th minute of the 3-1 win over Sunderland on Saturday and defender Smalling was thrilled to see him again on top form.
"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."