Technically if his grandma dies before the fight, he doesn't have to fulfill that promise anymore.
But I mean come on, Ricardo is the goat of his category, he toyed with Miyata who wasn't very impressed with Sendo and is now completely powerless against Ricardo who is barely trying.
Sendo winning this would make no sense. I do like his mindset though, I wish Ippo had that fire inside him (he used to but he lost it).