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. 
It's nothing new anymore for FIFA to treat Barcelona is a special team, many matches in the past the referees were supporting Barcelona by not make a fair judgement, the decision is always pro to Barcelona. Not to mention robbery of Ballon d'Ors. So it's not surprising for FIFA to help Barcelona in terms financial.
I can see some teams may think that's great, but Barcelona is not those teams, they have like hundreds of millions of dollars just in debt, think of how rich you need to be to have that kind of debt. This is why I believe that they are going to do fine with this, Fifa is not doing something special for Barcelona.