Borthwick-Jackson, 18, has made only half-a-dozen appearances for Louis van Gaal's side and his only senior starts came at Bournemouth and in the FA Cup against Sheffield United.
He began Sunday's 1-0 win over Liverpool among the substitutes but was called upon by Van Gaal before half-time at Anfield when Ashley Young suffered an injury.
And it was Borthwick-Jackson's mature performance at the home of United's bitter rivals that convinced Smalling his defensive colleague can handle the spotlight.
"I think he showed he is ready for a stage like this," the England international told ManUtd.com after the Liverpool game.
"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."
Injuries have left United depleted in defence this year and Van Gaal has therefore had to use a number of inexperienced players at Old Trafford.
However, Smalling insists Borthwick-Jackson's mentality means he needs little guidance.
"I like to talk to the defence no matter who is playing," the 26-year-old said.
"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."
Source : PA