That's because Donaire doesn't struggle yet against any other boxers at 118 since fighting Naoya Inoue. Just for a recap, after suffering the defeat at the hands of Naoya Inoue on their first meetup, Donaire bounces back on his next couple of fights with an impressive 4th Round Knock Out at both, winning a championship belt and successfully defending it, respectively.
If Donaire won't be able to win against a boxer that's not even on the Top 5 rankings, it's time now to consider hanging his gloves. Not just because he can't able to cope now with the competition but losing this fight will also put him below rankings. If he continues to fight after losing, then maybe just for satisfaction only and extra money before retiring.
But before anything else, I don't think that Donaire will show struggling against Santiago. However, my concern is, the longer this fight is not materializing for real, the longer Donaire will remain inactive in the ring. Next month will mark his 1-year inactivity in the ring (June 7, 2002).