Extremely difficult fight to pick.
Pacquiao is going to be 41 years at the end of this year and has been in a lot of wars. Never mind the decline of his speed, punch volume, and explosive reflexes (things that have been going downhill for over 10 years), his appearance is of an over-the-hill fighter.
He is winning fights now via his vast experience, his Welterweight power (enough to command respect, but not fear), very awkward southpaw movement, and his intelligent aggression. But that's about it. His flaws (you can counter him by timing his lunging punches) seem to be far more obvious now.
That being said, Keith Thurman has never been a complete boxer-puncher in my evaluation. He ALMOST was...early in his career, but never really evolved. He is no where near the all-around fighter that Terence Crawford and Errol Spence are (2 fighters that can destroy your body on the inside, can outbox you on the outside, or can create counter-punching opportunities from mid-range). Thurman has always relied on his athletic outside footwork to create openings for his awkward looping punches (not surprising his injury was on his right elbow).
If Thurman comes in like he was in the Shawn Porter fight, I think his athleticism is too much for the older Pacquaio (Pacquiao is going to hesitate to attack as much). But that's the question. Can Thurman get back to that state physically & mentally? I'm going to lean towards youth, pride, physical talent for the division, and intelligence. I say Thurman wins a close UD. I'll even say that he catches Pac & buzzes / drops him, but then boxes him for the rest of the fight.
Thurman by decision.