His arrival was expected to see Forest build on two successive play-off finishes and push for promotion to the Premier League - but he quit on October 2 after less than four months in the job.
"I just felt after that [Birmingham] game with what was going on, something had to change drastically," McClaren told The Independent.
"I couldn't see that with the relationship I had by then [with the board] that I could affect that. For the benefit of the club and fans I thought [leaving] was the best decision.
"Ultimately the chairman has stepped down and I think in the long run the club will benefit, Frank Clark coming in was a very good decision.
"Steve Cotterill [McClaren's successor] has got experience of the Championship and generally I had no problem with the players. They will prosper going on now within a more settled environment which I never had before."