Marshall said that he was not happy at his previous franchise the Miami Dolphins and would certainly not have secured a new £17.8m, three-year contract extension.
"The transformation started in my life before I was traded," Marshall is quoted as saying by NFL.com. "I think it was career-saving.
"I don't think I'd be sitting in this position, talking about an extension. I probably wouldn't be having the success that I've had on the field. It wasn't right for me."
Marshall has played all 16 regular season games in both his years with the Bears.