Vallecano are holding a psychological advantage in hand because of that. In addition to that Real Madrid couldn't do better than drawing with Vallecano not long ago as well. Combining all of this it seems a little risky to bet on Barcelona here. 
Yes, maybe Gavi's injury will have a big effect on Barcelona. They rely on Gavi in the middle and he always featured in the first 11 players (starting eleven). Maybe Xavi should think about starting to look for new players because I got the news that Gavi will take a long time to heal. Historically, Barca have always struggled against Rayo, but that's just a statistic. There is always a miracle before the game is over. Barcelona might be able to bring home 1 point.