Zidane announced his intention to step down from the head coach role after just two and a half years at the club just five days after securing their 13th UEFA Champions League title.
Club President Florentino Perez has forged a reputation as a trigger-happy leader of Los Blancos due to his relentless drive for success and Capello feels Zidane was not prepared to be next in the firing line.
This has been widely indicated by the fact that Zidane has become the first manager to resign from the club during Perez reign at the LaLiga giants.
"Without the third Champions League he'd have been sacked by [President] Florentino [Perez]," Capello told Sky Italia.
"There was a lot of polemic and Perez wants to be the Coach. When he wins he's happy, if he doesn't win he blames someone else. I was sacked the same way.
"I don't think Zizou has another job lined up because there aren't any free clubs, I think after winning so much he wanted to show Florentino that he didn't have to coach, and he could decide for himself what he wants to do.
"Now he'll have to prove he can win elsewhere too, but he was one of the greatest Coaches in Real Madrid history," the former England boss added.
"His replacement? I'd say Guti, another internal choice after Zidane. Otherwise, there's only [Antonio] Conte or maybe [Maurizio] Sarri."