Big Sam hailed Nolan - signed in June 2011 from Newcastle for £3m - as a key factor.
He said: "Whenever a team is relegated, there is lots of disruption. There's no unity and you have to rebuild team spirit.
"That's why I always say Kevin was the most important signing I ever made. It's not what he does as a footballer - it's what he does as a captain. He unites dressing rooms with his experience and is a leader of players."