The impact will not be too big, Frenkie de Jong is likely to play as a central midfielder to replace Sergio Busquets, while Gavi has the potential to appear to reduce the impact of Busquets' absence in the Europa League opener against Manchester United.
Basically, Barcelona is quite ready with the potential midfield options they have this season. The absence of Sergio Busquets will not cause any deep concern for the team because Barcelona has prepared everything carefully before this
De Jong is able to play as a defensive midfielder, but if Xavi doesn't want to play him there then he can have Frank Kessie there.
While Gavi and Pedri can be played simultaneously as midfielders. I'm sure without Busquets Barcelona won't be a problem, but without Raphinha and Dembele it will be quite worrying.
Then ready or not ready for Barcelona to meet Manchester United, I think they still have to be ready. His recent good form can supprt good result to be hoped for, but still both teams need win which makes this match really difficult for both of them.