Given that the fight's outcome was close and Donaire was defeated, they should make the rematch on this very same year as every time goes
That's the big reason already for Donaire to retire if:
- the fight went close in favor of Santiago
- the fight went landslide in favor of Santiago
No need for rematch but rather hang the gloves. Santiago is not even on the contender list but was just given a chance for a title shot. Again, as I previously said, if Donaire can't keep up with Santiago, there's no way he can keep up now with those top-ranked boxers at 118.
I highly agree Donaire not winning over Santiago means his boxing career is over he might one over a less skilled boxer but as stated he will have a hard time winning against the elite boxer of the said weight category.
There is no need for a rematch for this fight if Donaire lose because Donaire will be much older and Santiago will have a better experience by that time.
Statistically wise, Donaire has all the advantages over Santiago.
Some of these are height advantage, reach advantage, better KO percentage, much faster, much quicker, having a strong punch, good at counter-attack, versatile, agile, better footwork, and what else, much good looking, just kidding.
Santiago should not be a problem for a 40-year-old Donaire since Moloney, the supposed original match is even considered much tougher than Santiago.
Statistics will be nothing if old age catches up on Donaire. This is one major reason why legendary boxers even though have better statistics losses to the young ones. And one of the reasons why Manny Pacquiao won over Oscar Dela Hoya.