Harlequins have tied down “one of the most sought-after players in Europe”, according to scrum-half Danny Care.
Last week, 18-year-old a four-year senior contract to stay with Quins until 2022.
And Care says Smith, an England squad member, would have turned down big offers to stay at The Stoop.
“I’m sure other clubs were throwing some serious money his way,” said Care, a co-presenter on BBC 5 live’s .
“He is a guy you can build a team around. He runs the game so well already – he’s a natural leader and a natural talker.
“He would have been one of the most sought-after players in European rugby at the moment.”
Smith’s first senior professional contract will see the fly-half earn a substantial salary increase, but Care said the teenager would stay focused on rugby.
“He’s a great kid who has his head switched on. All he wants to do is be a brilliant rugby player, and he wants to get better every day,” Care added.
“He is so eager to learn. You see him in the England camp and he’s just in Owen Farrell and George Ford’s ear, just trying to learn and better himself.
“He’s got to be a young lad as well and he’s got to enjoy himself – and he will, off the field.
“But he knows his bread and butter is playing rugby. If anyone can deal with making that much money, it’s Marcus.”
Latest Vunipola setback ‘devastating’
Smith is expected to be included in England’s Six Nations squad, which head coach Eddie Jones will name on Thursday.
But while England have a number of options at fly-half, Jones faces a selection dilemma in a handful of other positions, especially loosehead prop and number eight.
Billy Vunipola’s a fractured arm, coupled with Nathan Hughes’ knee problem, leaves England desperately short of experience at the back of the scrum.
“It’s devastating news for Billy, I’m gutted for him. I know how hard he has worked to get back,” Care said.
“He is one of the best players in the world and as an England player I was really looking forward to getting back playing with him, and I am sure every England fan wanted to see him in a white shirt.”