Simpson helped Roy Keane's Black Cats win the Championships last season and said: "I am a Manchester boy.
"I know Roy would have liked me to go back on loan but the manager said he wanted me here and see how the first few months of the season goes.
"He wants me to keep working hard and he will try to give us some games."
On working with Keane, Simpson added: "He worked under two of the best managers of all time in Brian Clough and Sir Alex Ferguson, so it would be a bit of a surprise if he hadn't taken anything from people like that.
"He is certainly completely different as a manager than he was as a player.
"He is quite calm. He thinks about everything. He assessed everything in training. I am sure Sunderland will do well this season.
"I used to watch him as a young boy. I had a few chances to train with him when I was younger and I have grown up working with him.
"For Roy to take me to Sunderland for five months was fantastic and it really helped me a lot."