Pac is old and past prime, looked like **** on his last fights but I say he would still win and probably look good doing so.
If this is the Prime 28-30 years old Pac, then Pac will have too much stamina, too much speed, too much combination and tazamnian relentless in and out side to side blitzing round after round. Rios will make it last but will be worn out by cuts and bruises and Pac will win by mid to late TKO. If Rios however trades and goes in without head movement, he gets KO'd early. Now for Rios vs current Pac, i would say Rios has a chance but I would say slim. If he can however put enough pressure on Pac and if Pac no longer has that stamina to sustain footwork and combination then Rios might just outwork Pac and win a UD.