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.
which of course determines the assessment that occurs, because the pre-season match can be said to only measure how strong the players are until what is desired until what is expected happens and of course the coach assesses all of that before being deployed for a more decisive one. The point is that everything needs to be assessed as early as possible to maximize facing the next match.