Production sources tell TMZ ... the folks who run the popular Netflix show are scrambling behind the scenes to figure out who is going to step into Christine's place as the bad girl, and they're hopeful holdover Chelsea Lazkani leans into the role in upcoming seasons.

We're told execs approached Chelsea about playing into the villain role once it became clear to producers in June Christine wasn't coming back, leading to conversations with Chelsea about embracing the role Christine played so well.

Our sources say producers understand Chelsea, a British real estate agent who joined the show in season 5, has big shoes to fill here ... but they're hopeful she can step up to the plate.

Other candidates, we're told, include newcomers Bre Tiesi and Nicole Young ... they're joining "Selling Sunset" for the upcoming seasons, and sources say there's a possibility producers will look to one or both to be the next Christine if Chelsea doesn't cut it.

Filming began this week, but we're told it's too soon to know if producers have their villain just yet.

TMZ broke the story ... Christine and the producers reached a mutual decision for her to leave the show and she's moving onto other projects, signing with IMG models.

Jason Oppenheim told our photog Thursday he's not stressed about Christine leaving, saying he has no concerns her exit will hurt the show.