In theory a team can improve during the season is what you said. Of course, a team can improve during the season as it can decline too.
How is it fair or realistic to expect from Unai in a month of pre-season to drill his team of players he has never worked with before in a new nation league and culture to be ready for beating Manchester City who are a drilled team for a few seasons now under Guardiola? Or Chelsea who are still unbeaten too?
Other managers drill their team for a season and are expected to get results the season after, definitely regarding beating top teams. You tell me Unai had a pre-season to get the job done.
Come on man.