The matches that are played now, from pre-season, are just friendlies, nothing exciting
Furthermore, these are matches that make little sense, given that many coaches experiment with things they otherwise couldn't do.
different modules, different formations, different strategies
things you can't experience in the league or cup
so there is no point in considering them
Yes, preseason matches sometimes don't provide a clear picture of a team's true performance. Last season, I often saw teams perform poorly in preseason but then perform well when the season started. And vice versa. So, we can't even judge a team's overall ability based on preseason alone.
However, for teams with several new players, the coach will usually test them by placing them in different positions, where they will then assess their suitability. And sometimes, new players don't perform as well as expected, as is the case with Gyokeres at Arsenal right now. But again, this is just preseason, and what we'll truly see is when the season actually starts.
I am not sure that Gyokeres will be able to perform as many people expect because the Premier League is a different beast from the Portuguese league, and in Portugal everybody was playing for him. At the same time we can't judge a player who just arrived at a new team, especially during the summer friendly matches where the result doesn't matter at all.