True, the adaptation process takes time. Every player has different qualities and they need different times to adapt well to their new team and friends. Not to mention if they don't really understand the coach's strategy and don't have a good relationship with their teammates. If during the adaptation period they don't get good support from their team mates, then of course these players need quite a long time to get into their best form.
There are some players who immediately fit into their new team and some who don't. Of course there are many factors that affect how fast or slow a player adapts, but in most cases I think the most important factor is the support of other players.
And I think it all depends on the personal qualities of the player himself... This is a professional sport and players regularly change clubs, teammates and coaches... Of course adaptation happens and there are probably psychologists in the team and a certain period for the player to adapt to the new club... But it seems to me that there is nothing difficult for a professional athlete...
Even though they are professional athletes, the adaptation problem really depends on their new teammates and coaches and game strategy. I remember Nunez having a hard time finding his form at Liverpool, Coutinho having a hard time finding his form at Barcelona, Hazard struggling at Real Madrid and all three players are professional athletes who have been recognized as star players.
Coaches and teammates can help the adaptation process of a new player. If his new teammates don't like the new players, then they must be having a hard time in their new team. Lionel Messi is also having trouble getting his best form at PSG, that's also because his team-mates don't support his adaptation 100%.