I do see that Donaire will win this fight as he's facing a not ranked boxer here. Not degrading Santiago here but there's no big achievement yet he did even though he's currently at the peak of his career.
In this case, he can still show his skills and promote his next fight with lots of sales because we all know, he still has that flash speed and huge power when it comes to kinds of opponents like Santago. It's really hard for Donaire to retire because as of today, there is no Filipino boxer who is more famous than him who is still active because others are just a bunch of still climbing the ladder. He worked hard to get to where he is right now and to fully use it to his advantage until the end, it's just one of the wisest decisions in his life that he could ever make. Maybe he still can fight for another 3 or 4 fights if he easily wins this one.
This is why this fight is so important for the future of Donaire, a clear win will dissipate all the rumors about a possible retirement and his fans will be more at ease knowing they can still watch Donaire fight professionally for a few more years, but a difficult win or a defeat will only increase those rumors, and while I think Donaire has enough to beat Santiago it is not clear to me if he still has a high enough skill to win easily due to his age.