McDermott was axed in March with just nine games remaining following a dismal run of results that left the club joint-bottom of the Premier League.
The club has taken a lone point under new boss Nigel Adkins and languishes 10 points from safety with just four games to go but Madejski insists owner Anton Zingarevich made the right call.
"Anton is football mad but it wasn't a spur of the moment thing," Madejski told Reading's official website.
"Everybody knows it's all about winning. If managers don't win they can expect to leave in spite of any previous record; it's why they get paid good money.
"I'm very happy with Nigel Adkins, he's very enthusiastic and motivating and I'm delighted Anton owns the club because it's not for the faint-hearted."
On Reading's perilous position, he added: "It's not over until the fat lady sings and she's not singing just yet.
"Of course it would take something very special indeed for us not to be playing in the Championship next season.
"Funny things happen in football and if I don't have the belief as Chairman then how can I expect anyone else to have it? So I'd never condemn Reading until anything is confirmed."