Borthwick-Jackson was unexpectedly thrust into the action after 40 minutes at Anfield after Ashley Young picked up an injury.
The youngster slotted in at left-back and acquitted himself well as Wayne Rooney fired in a late winner. Smalling believes Borthwick-Jackson has a bright future ahead of him.
“I think he showed he is ready for a stage like this,” Smalling said.
“He has been drip fed quite a few games this season and he has done very well. To be able to stand up here and perform like he did shows he is ready for any game because if you can perform here then you can perform anywhere in the league.”
When asked if he looks after Borthwick-Jackson on the pitch, Smalling said: “I like to talk to the defence no matter who is playing.
“Maybe it’s a case of spurring him on when he does something good just to keep that going but on the whole he looks after himself because he is a very level-headed guy.”
Borthwick-Jackson has been dropped in at the deep end at Man United this term following a number of injuries at the back but the 18-year-old has showed a maturity beyond his years.
It was only his sixth appearance for the United first-team but he can be proud of his performance against United’s fierce rivals. With the likes of Luke Shaw and Marcos Rojo set to be sidelined for a while yet, Borthwick-Jackson should get plenty more opportunities to impress.