Rob Kardashian & Lamar Odom -- Rob's Problems Triggered By Lamar Leaving Family

Rob Kardashian's downhill spiral was triggered by Lamar Odom ... but we've learned it had nothing to do with Lamar's drug use ... it was all about fatherly abandonment.

Sources connected with the Kardashian family tell TMZ ... Rob was devastated when his dad -- famed O.J. lawyer Robert Kardashian -- died back in 2003. We're told he took it much harder than his siblings and never really got over it.

When Khloe married Lamar ... the bballer became a father figure to Rob. Sources close to Khloe tell us Rob thrived around Lamar, inspiring him to start a business and get in awesome shape. They became best friends, but Rob idolized him.

Khloe has said on the Kardashian's show, Rob's recent troubles were triggered by the dark things that happened in the house. But we're told it's not about drugs -- Lamar compartmentalized his drug use and mostly kept it out of the family home. We're told Khloe was referring to the arguments and breakdown of the marriage.

Our sources say when Lamar cut off the entire family and went radio silent ... Rob went into a tailspin because he felt abandoned by the only person who measured up to his dad. And that's when Rob got deep into lean and other drugs, which resulted in a dramatic weight gain.