Scunthorpe boss Knill delighted with development of Everton's Duffy
Knill believes the Irish Under-21 international is improving all the time in his first extended taste of first team football.
"I think he's been excellent for us, it's easy to forget that he's so young, I think he's going to be a really good player," said Knill.
"You have to talk to him properly. There are certain ways you can go about it but if you speak to him and show him he will continue to get better.
"I think his performances are getting better. The biggest thing that any centre half needs is, when the ball comes into the box a desire to get your head or a foot on it, and he has that.
"He has a great chance to be a really good defender. He has good feet and I'm delighted for him."