Resume wise Greb smashes anybody near his weight, in history ... more or less. But as we all know styles make fights, RJJ was a superlative boxer and there's no footage of Greb, so one couldn't honestly make a pick in good faith.
I'll go with Roy here. And I dislike Roy. Greb for all his speed and skills was a little on the short side and not a puncher at all. Jones is one is one of the most skilled and fasted handed fighters ever filmed. And unlike Greb he can hit. Oh but what about Greb's defense?! My reply: Jones had no trouble making Hopkins and Toney's defense look ordinary. While Greb's movement and ducking were a little different, I'll side with Jones timing to land his share. I don't have most of Greb's scorecards, but he wasn't a master shut out type vs the best. Walker I believe was in the lead until the final rounds. The best way to beat Roy Jones is to have some power and make him retreat and become punch shy from the incoming. Jones UD over Greb based on the way they match up.