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.
Gavi was showing a decent performance before getting injured. So his absence would be felt of course. By the way I have read interesting news about him recently. It was saying that FIFA were going to pay Barcelona 20k euros per day during Gavi's injury. Is this really true? If it is then I must admit that I'm hearing about such thing for the first time in my life.
Gavi and Pedri both are very valuable players for Barcelona now. They are Barcelona's future so Barcelona are also doing their best to have a long and strong bond with them.