You don't "know" it but you can be pretty damn sure that Pac that far declined isn't going to be able to mix it with the very best. If you really think that version of Pac would've beaten Spence then I don't really know what to say to you - way too far off the pace. At his best, he'd have beaten Spence, but he's not remotely close to that now. If that had been an uninjured Spence out there then best case for Pac is about 10-2 loss, but if we're honest he'd be lucky to see the end.
Ugas won, and I think almost everyone was rooting for Pacquiao in this fight which strengthens the claim that the decision was just.
You'd have to either be a Cuban defector yourself (in that unique crosshair of being culturally proud of your tribemate but also being anti-nationalist regime to not consider him a traitor) or just a longtime hardcore Pacquiao hater to have been rooting for Ugás. So like ~2% of boxing fans at this point, since the former are not a statistically meaningful group and the latter have mostly either drifted away from the sport or had their passions cooled since the days of the big Mayweather rivalry.
Pretty clear decision. The staggering difference in connect percentage tells the whole story of the fight. Pac threw a lot but wasn't very effective or landing much. Even in the early rounds when he was blitzing Ugas, those flashy 3-4 punch combos were mostly missing or being blocked and he only landed maybe one clean shot. Pac knew this and was trying to win rounds on aggression and work rate, but the numbers don't lie. Ugas from the very first round was bothering Pac with his long jab. Then he picked up the pace and gained more and more confidence from round 5 onwards while Pac started slowing down and becoming more tired. Ugas right to the body and Cuban right hook landed over and over again and Pac never adjusted, blocked, or moved his head. Gone were the in and out movement and tricky angles Pac used on previous opponents, he just kept trying bum rush and overwhelm a taller rangier skilled opponent and that rarely works unless you have an overwhelming advantage in power and boxing ability which wasn't the case here. Everyone underestimated Ugas' technique and ability to remain calm under pressure. Had he panicked and opened up (which is what Pac wanted), he would have abandoned the tactics that got him so much success and Pac would easily outland him. Overall it was still a fairly close and entertaining fight, but Pac is just too small and old for such a tall cagey opponent with good accuracy. Perhaps if Pac wasn't so aggressive and used more head movement and lateral movement he could have edged it, but it most definitely wasn't some sort of robbery.