The Scottish fullback rejected new terms at United to move to Preston this week.
Ferguson told pnefc.net: "They offered him a new contract at United, so that's how much they rate him. The lad wanted a fresh start, he wanted to start playing first team football so were pleased to jump on that and it was done pretty quickly.
"He's got a good pedigree, he has been brought up at a fantastic club like United, he comes highly recommended, they think highly of him. He's got good character, good attitude, he came in and trained really well and fitted in straight away."