Bas by his own admission had awful TDD. He conceded takedowns left and right. However, his ability to use an active guard to scramble, improve (or even reverse) position, defend & attack submissions, or at the very least minimize the damage he received while on his back was really quite exceptional for someone from a karate/kickboxing background and the era he fought in.
I think a big part of the outcome of this hypothetical fight, believe it or not, might actually end up being the enclosure. Randy struggled in the ring while Bas did just fine there. Conversely, cage-fighting was always Couture's wheelhouse because of his reliance on the clinch. I'll assume we're going with the cage on the presumption that this scenario is taking place in an alternate timeline where Randy didn't run off to Japan and had instead remained to defend the UFC Heavyweight title against Bas. So cage it is.
I think Rutten is able to make it more than competitive on the feet (and would probably win if it stayed there)... but ultimately Randy encroaches on his space, presses him to the fence and dirty boxes him before taking him to the canvas and controlling him. Randleman was able to do it consistently without being submitted or reversed and that led to a fight that many people believed he won. The only reason he didn't get the nod is that he didn't do enough damage, whereas Bas constantly threw up strikes from the bottom.
I think Randy -- who outwrestled Randleman despite lacking Kevin's raw strength, explosiveness, and athleticism -- would have been savvy and well-rounded enough to put the beating on Bas once he had him down via GnP and possibly even submission attempts of his own (they'd fail, but would look good to the judges and give Bas something to think about). Not to mention that he likely would have earned some points from blows landed in boxing range prior to the takedowns.
Bas would be a live underdog though: always a chance of him landing clean on the feet as he catches Randy coming in or managing to snatch up a leg lock/guillotine on the mat.
I'd say 65% in favor of Randy by Decision or late TKO, with 35% Bas by (T)KO or sub.
