I would say yes he is worth that much to Chelsea.
Lampard will be looking afresh at the cb situation at the club and if you think Rudiger, although very injury prone, is possibly a certainty then you're looking at his partner.
Sideshow Bob (Luiz) is a catastrophe waiting to happen at that level, despite being comfortable on the ball, he has a touch of the John Stones about him, he's also not getting younger and perhaps he should have learnt by now.
That leaves Andreas Christensen who was first reserve last season, useful benchman and needed for Rudiger absences.
To me it's French international, relatively young and improving Zouma vs Sideshow Bob the accident waiting to happen.
Zouma looks to have every chance of being a first choice at Chelsea to me and I can't really see them entertaining selling him. with today's inflated prices it could take a huge offer well north of 24m for them to even contemplate it for a minute.