Pundits are calling for the 19-year-old to start in the Republic's crunch clash with Switzerland on Tuesday after an explosive cameo against Georgia.
"He's been brilliant," said Duffy.
"I'm looking after him. He's a great kid. I'm just trying to keep him settled, keep his feet on the ground. I'm delighted for him.
"If he starts against Switzerland? No problem for him. He's mature for his age.
"He's playing in the Premier League, you all seen him against Tottenham.
"He thrives off big moments, he thrives off big games and that's what he wants. He loves scoring goals and he always will, whatever level he plays at and hopefully for Ireland.
"But you can't put too much pressure on him, he's only a kid and he's played three or four games in his career.
"Let him ease into it and hopefully, in five or ten years' time, we're talking about a great player."