Khloe Kardashian -- In the Dark About Lamar Odom's Rehab

Khloe Kardashian is totally unaware her estranged husband Lamar Odom entered a rehab facility -- despite the fact she lead the charge for him to get help in the first place ... TMZ has learned.

Sources close to the couple tell us Khloe never got so much as a heads-up about Lamar's plans, and was completely blindsided by the report he's checked in for treatment.

We're told one big reason for that is Khloe and Lamar have not been talking at all since she issued an ultimatum last weekend -- get help or our marriage is over -- intending to force his hand on the issue of rehab.

As we previously reported ... Lamar started a treatment program last summer in San Diego, but bailed after only 3 weeks.

One source says although Khloe's been kept outside the loop this time -- she does hope wherever Lamar is ... he's actually getting professional help.