Preston manager's comments on Armstrong signing:
"He's young in age but he's mature in terms of his mentality, how he wants to succeed, his performances are already mature on the pitch," said Heckingbottom.
"It's not one where it's like a last minute, take the player - we like Harrison and we wanted to get him and beat a lot of people for him, it was just a case of will Everton let him out?"
The 18-year-old made 15 appearances for Derby in the second half of last season and will be looking to build on those minutes at Deepdale.
"When he played at Derby he played deep, he can do that, get on the ball, break things up and compete defensively," said Heckingbottom.
"I think we can get more out of him in terms of getting on the front foot and press high and getting in the box, we want goals from all over the pitch."