From my experience, some players start slowly on joining new team. The poor form may start since he was at previous team. Usually I do not use a new player in a hurry. If he played badly, I will only play him in friendly matches until the form improve. It is quite often their performance will turn around in next season. Maybe due to changing in team formation, or role assignment like free kick. Certainly, some players' performance never improve before I sold him.

BTW, I usually has less patience for goalkeepers, because they cannot be replaced with players in other positions.