The Irishman started his career at Manchester United at the young age of 17, and after 13 seasons at the club he and Sir Alex decided it was the right time to leave.
“I always said the manager would decide whether I would stay or leave. We evaluated things and he said he couldn’t promise me as many games this season as in previous seasons. I had just turned 30 as well, so it was time to get playing week in, week out and come to a fantastic club. ” O’Shea told The Times.
The defender played over 250 league games for United before deciding to head to Sunderland this summer along with team-mate Wes Brown.
“I know it’s one of the biggest and best clubs in the world, but if you don’t feel you are contributing on the pitch then it’s right to go,” O'Shea added.
The 30-year-old is looking to the future and he wants to bring his former club's winning mentality to the Stadium of Light.
“Every competition United go into they are expected to win, that’s what drives them and that’s why they have been so successful,” O’Shea said.
“That’s the simple difference between the two clubs, but we need that mentality here. If we don’t improve, it will be seen as a disappointment. You have to want to do better.”
O'Shea made his debut away at Swansea last week but will miss Sunderland’s home game with Chelsea on Saturday afternoon through injury.