Herschel Walker -- Cowboys Should Sign RG3 ... 'I Know He Can Play'

Herschel Walker says he still believes in Robert Griffin III -- and says he'd LOVE to see his former owner Jerry Jones sign the QB to the Cowboys, ASAP.

"I know RG3 can play," Walker told TMZ Sports ... "[he] just gotta find a team that's gonna believe in him."

Walker says his Boys would be the perfect fit -- though it's unclear if he likes RG as a starter or a backup ... especially with 35-year-old Tony Romo coming off a pretty serious injury.