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. Xavi can look for options in the transfer window later or there are several players who replace Gavi's position, I think indeed several players can still be relied on by Xavi.
It's not an accident that Barcelona are statistically ahead of Rayo even though their H2H is more difficult because it's an away game so of course we're thinking about two possibilities in this match.