It wasn't even close. 
Brandon Rios didn't belong in the same ring with Manny Pacquiao, who absolutely annihilated "Bam Bam" from start to finish for 12 rounds. Pac-Man came away with a resounding unanimous decision, via BoxRec.com, 109-119 (Lisa Giampa), 108-120 (Michael Pernick), 110-118 (Manfred Kuechler).
After the fight, he made it clear he is still as passionate as ever about his desire to compete, as relayed by HBO Boxing:
Pacquiao showed up with fire in his belly against Rios, who entered the bout as one of the world's top lightweight contenders, having lost just once in his career before getting walloped by Pac-Man.
After losing his two previous fights, the feisty Filipino boxer needed a big win like this one to rejuvenate his career, and he didn't fail the test. 
Truth be told, the criticism he received after losing to Juan Manuel Marquez was rather unwarranted.
He should have won the bout against Timothy Bradley, but a controversial (and wrong) split decision gave him his fourth career loss. Furthermore, as Steve Kim noted after Saturday's fight, he was looking great against Marquez before getting knocked out:
There is no doubt many watched Pacquiao easily take down Rios this past weekend and thought, "Wow, Pacquiao is turning back the clock tonight."
The truth is, he never really took the fall that was portrayed in the media. He's still the same fighter he was when he rattled off 15 wins in a row leading up to the Bradley debacle.
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Rios was no match for Pacquiao, which means there aren't many boxers who are.
Chris Mannix of SI.com thinks a fifth fight against Marquez is likely upcoming, and there's no doubt people would shell out pay-per-view money to watch that fight:
Another intriguing option would be a rematch against Bradley. Everybody knows Pacquiao was jobbed by the judges, and it would be satisfying to see him get his revenge.