it's based on seniority. the moment a judge is appointed to SC, their date of retirement determines whether someone becomes CJI or not.
in the wiki page itself, you can see who will come next after justice khanna.
List of sitting judges of the Supreme Court of India - Wikipedia
en.wikipedia.org
I think recommendation by CJI means little, GOI strictly goes by seniority.
In fact, tradition within judiciary is to recommend the most senior. It's more a tradition than anything else.
