In my personal opinion, Real Madrid will not field their core players against Real Betis, Carlo Ancelotti will definitely give their back-up players a chance, because Ancelotti will never take risks, because there are still very important matches waiting for them, they have to face Liferpol in the Champions League final, Real Madrid wanted to win against Real Betis, but they didn't want to take any risks, especially until their players got injured.
Real Madrid has wan the league already, Real Madrid do not need to win the last match, but I doubt even if Real Bestie would win the match. What Real Madrid should and is preparing for now is Champions League final as it will be meeting with Liverpool in Stade de France. Real Madrid may want to win Real Bestie but it is just a match that is not necessary for winning, but at least, I am not expecting Real Betis to win. It is good that Real Madrid coach is not making use of his main players, resting them for bigger competition that is not far from now.