Raiders legend Bill Romanowski says he's got a bad taste in his mouth over the Khalil Mack trade ... telling TMZ Sports Oakland's gonna REALLY regret giving Mack away.
"This is a guy that comes around, like a Lawrence Taylor, once in a lifetime," Romo says.
Remember ... Khalil and Jon Gruden never even spoke after Chuckie took the head coaching job -- and Gruden decided to punt the All-Pro to the Bears instead of working with him earlier this month.
Mack's first game with Chicago went well -- a sack, a pick AND a TD -- and Romanowski isn't at all surprised that the Raiders are feeling some immediate remorse.
In fact ... Bill seems downright pissed over the move -- even questioning how Oakland was able to get Derek Carr's mega deal done so quickly but couldn't get one for Mack.
"For Raider nation, you got one of the best defensive players in the National Football League -- figure out a way!" Bill says.
"Hey, you got Carr signed, figure out a way to get this guy signed!"
Romo added, "Sad day for the Raiders when that went down."