McDermott lost his job in March following a poor run of results and was swiftly replaced by recently-sacked Southampton manager Nigel Adkins.
"Anton is football mad but it wasn't a spur of the moment thing," Madejski told the club'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."
The Royals face an uphill struggle to keep their Premier League status as they sit 10 points away from safety.