The teenage defender is attracting interest from Manchester City, Everton and Newcastle United.
Evans, speaking with the Rotherham Advertiser, knows the Millers have something special in the 16 year-old centre-half.
“Harrison has been outstanding when he's been up with us at Roundwood in training,” Evans said. “The senior lads played against the youth team on Monday in a practice game and Harrison was outstanding.
“I would recommend that we keep him. I think he's going to be a really good player. I know that the club had two offers for him a number of months ago.
“The chairman said: ‘No. If we want to keep our young talent, we'll try our best to do that.’ It never got to a level where it was more sensible to take those funds and help the academy. Our academy chief endorsed that.
“We'd love to get Harrison on the pitch in some competitive League One games.
“We must not stop the pathway for any of our youth players if they're good enough. We need to be stepping them up.
“The younger lads who've been here longer and are a little bit older ... if they're not going to help us in the short, medium or long term, we'll move them on in the summer.”