Young's season at United finished on a low after they missed out on the title to bitter rivals Manchester City.
"To lose it on goal difference with the way City scored in the last couple of minutes was hard," he told the Mirror.
"Sir Alex Ferguson has been a massive influence on me. I said at the time that as soon as Sir Alex comes knocking at the door, it's a no-brainer. I was always going to go there.
"The first thing he said to me was that it was going to be a challenge because everyone wants to beat United. Everyone wants to be able to say they've beaten United because it's the biggest club in the world.
"I saw that as soon as I stepped in the building. It's been fantastic to be able to work under such a successful manager and hopefully we'll be able to bring success next season."