"James was outstanding," Smith told reporters. "He's 23 and is still just a kid who gets up in the morning and puts his trousers on one leg at a time like everyone else.
"A lot has happened quickly to him but he's settled in straight away and showed his class today. The boys enjoyed playing with him today.
"We see him as 15, even though many see him as a jack of all trades who has played right along the back line. And he wants to establish himself as a full-back."
London Irish travel to face Newcastle Falcons in the LV Cup on Sunday.