I totaly agree that barcelona beginning of the season was terrible and if xavi was there from the beginning of the season things would've been different and maybe barcelona could do something -snip-
Don't imagine something that can't happen.
Although Xavi was in Barcelona from the beginning of the season, Barcelona didn't automatically win La Liga title or UCL title. You must remember that Barcelona failed to win Europa League although Xavi already become the coach. It is a proof that even Xavi managed Barcelona since the early season, we can't expect too much they will win every competition.
Even though Xavi is a good coach, Xavi still needs time to restore Barcelona's glory. Moreover, Xavi was faced with Barcelona's financial condition
which was still not stable, so Xavi not only had trouble bringing in new players, but Xavi also struggled to keep several Barcelona players whose
contracts were about to expire. Like Osmane Dembele who is likely to leave Barcelona for free, after several times rejecting a contract extension
from Barcelona. Not to mention Gavi, who is a young Barcelona player whose contract expires next year, until now still have not reached an agreement
to extend the contract, because Gavi's agent was not satisfied with the contract offer that Barcelona gave. If no agreement is found there is
a possibility that Gavi will move to another team. So Xavi's task is very tough for next season, there are many things that must be done by Xavi
to make Barcelona perform well next season. So as you said don't expect too high on Xavi, but I'm sure Xavi can make a Barcelona performance
good next season. But to win the trophy next season, there is no guarantee Xavi can make Barcelona win the trophy next season.