Sounds like he is a quality CB. If that is the case then we should just keep him and have him replace Kjaer. He has all the requirements for an "elderly" CB checked off. Approaching 30 years of age, knows the team well, knows the league well, does not have to be a star but gets the job done every time (i.e. no errors that lead to conceded goals but also nothing extra).
If we look for a new CB it must be a young one, anything older than 25 years of age is not acceptable. We would be just fine with Thiaw, Tomori, Simic, a newly signed promising CB, and himself in the 3-CB defense which Conte will probably deploy. I do not think Kalulu will ever resume playing at a high level due to his injury. I honestly think Kalulu's career is over.