Bridcutt had not played for a month after trying to force through his transfer from reluctant Brighton.
He told the Evening Chronicle: "It was one of the hardest things I have had to go through in my career.
"When you are given an opportunity to come to the Premier League and to a great club like Sunderland, with a manager like Gus, you are not going to turn it down.
"It went on for a month and was only done in the last two days of the window.
"It was difficult. At one stage, I didn't think it would happen."