The pair played together at Swansea City.
“I found out a couple of weeks ago that Newcastle were interested. But it was really in the last two days of the window that it became something more intense," Fernandez, who also rejoins former Swansea team-mate Jonjo Shelvey at Newcastle, said of his move to United.
“I spoke with Ki a lot before I came. He only had good things to say about the club, the city and everyone at Newcastle.
“When I was on my way [to Tyneside on deadline day], he sent me a message telling me he was glad I was joining him. Everything was fine, we have a very good relationship."