Earl Thomas Will Regret Flipping Off Seattle Sideline, Says Ex-Teammate

He may not regret it now, but Earl Thomas will end up wishin' he didn't throw the ol' single barrel salute to his team last Sunday ... his ex-Seahawks teammate Walter Thurmond tells TMZ Sports.

Earl had been SUPER pissed about not getting a new long-term deal from Seattle this offseason, but decided to play anyway ... and promptly broke his leg against the Cards.

That's when Thomas -- bitter about the whole situation -- threw up the one-finger salute to Pete Carroll and Co. ... something Thurmond says the All-Pro safety will regret.

"That's one of those things where it's, 'I shouldn't have done that. I let my emotions get the best of me,'" says Walter, who played with Earl in Seattle for four seasons.

"But again, it just shows that we're just human. And this whole situation, it's a human aspect."

Don't get it twisted ... Walter DEFINITELY feels for his boy -- and says he wishes NFL teams would treat its employees more fairly.

BTW ... that Legion of Boom defense Thurmond and Thomas were a part of from 2010 to 2014?? Walter tells us it'll go down in history with the Purple People Eaters, the 2000 Ravens AND the Steel Curtain as one of the greatest EVER.