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.
Ferran torrest or raphinha can play at the left wing position. The rotation will be made by xavi. Im sure xavi knew the best for the club since there was a new about gavi got serious injury.
Many people are feeling worry caused by their favorite players might be affected by this. It's a just matter of time until everything will be fixed by the coach. We can only trust xavi who knows better than us regarding the situation of club.
It's time for yamal to show his performance in the main squad. We will see what kind of line up will be preferred by xavi. barcelona has not yet faced the strong team in UCL.