The 23-year-old striker joined the Tigers in a £1million deal on Tuesday and admits the presence of former Red Devils no.2 Mike Phelan helped get the transfer across the line.
"Mike Phelan was a big factor," Keane told the club's official website.
"I spoke to him a few times prior to signing and I think he had contact with a few people who are still at United.
"I've worked with him quite a bit. He was assistant to Sir Alex when I made my debut and he was really good to me. I'm looking forward to working with him again."
Keane, who could make his Tigers debut against twin brother Michael against Burnley at Turf Moor next weekend, added: "It's a fresh start and a new challenge.
"I'm 23 now so I wanted to come here on a permanent deal to get settled.
"I had been (at United) for a long time so it's an end of an era for me. I felt like my time had come to an end there.
"I couldn't see a path through to the first team so I made the decision to come here."