NFL Draft -- Randy Gregory 'Not Doing Well' ... After Draft Slide

Ex-Nebraska defensive end Randy Gregory is angry, distraught and "extremely disappointed" after not being selected in the 1st round of the NFL Draft ... TMZ Sports has learned.

Multiple sources close to Gregory tell us Randy was POSITIVE he'd be selected on the 1st day of the draft -- which is why he agreed to go to Chicago and attend the draft in person in the first place.

Of course, there was buzz his draft stock had fallen ever since he tested positive for weed at the NFL combine. Before the combine, some experts said he could have been the #1 overall pick.

So, when the first 32 picks went by last night ... and his name wasn't called ... we're told Randy took it especially hard.

"Randy is extremely disappointed. We thought he'd drop, no one predicted he'd be out of the first round," one source tells us.

Another source close to Gregory says, "Randy is a little distraught over last night and didn't even imagine this happening, but he's keeping his faith."

The good news for Randy ... our sources say the draft slide is only motivating him to work harder.