Demarcus Lawrence went OFF on fans who criticized him after the Dallas Cowboys took another L on Sunday ... telling his haters straight up, "F**k all ya'll."
The star pass rusher -- who was limited to no sacks yet again in Big D's embarrassing 49-38 loss to the Cleveland Browns -- took a TON of heat on his social media page after the defeat.
But, instead of brushing it off ... the 28-year-old went into attack mode -- and ripped every single critic in a scathing comment.
"I play my heart for the Cowboys and my brothers," Lawrence said.