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.