The former Royals manager was fired last month, with the club's owner citing a lack of signings as part of the reason for the dismissal.
McDermott, who is now in charge of Leeds United, told the Daily Mail: "I know the owner said I didn't buy any players in January, but we couldn't agree fees for Thomas Ince at Blackpool and Gylfi Sigurdsson at Spurs. We made big offers for them.
"If I had been here a couple of games earlier you never know, but I was brought in with the brief to make sure we didn't get relegated. All I know is that I have had the backing from the owners GFH Capital to make the changes I want at the club to get promoted.
"At Reading we had players who could hurt teams and I want the same at Leeds. This club is proper."
McDermott's Leeds side face Birmingham City this afternoon.