Firstly, the only way for Fujitora to sit on sidelines is if Oda decides to pull another Jack on us.
Secondly, not that I don't want to see Zoro vs Kizaru, but the only way I see it happening without Oda pulling another Jack is if the fights against the Marines take place in two waves, with two different purposes.
First wave (pre-Raftel) ends with Luffy vs Ryokugyu and Zoro vs Kizaru in order to save Sabo and Vivi.
Second wave (post-Raftel) ends with Luffy vs Akainu and Zoro vs Fujitora in the Marines's last ditch attempt to demolish the Pirate King crew.