We raised this question of positions recently in another article.
Mourinho had following to say as reported by DailyMail–
I know Ivanovic is very comfortable in the centre but I see him and Cesar Azpilicueta as our two right backs. Our defensive group will be good.
So this pretty much confirms that Tomas Kalas will be the CB backup this season. And Iva will fill the role in any emergencies only. Maybe when next season Wallace returns from loan, the equation may change.
This also implies that Azpi will have play second fiddle to Iva atleast for the start of the season.
Mourinho also said that he saw Luiz as CB which is something everybody knows!