I want to say The defeat was purely due to Mauricio Pochettino, wrong formation, Chelsea played four at the back in pre-season and goals were scored, the league started and he started to change it, two attacking players against Forest at home? Mauricio Pochettino better fix it soon or things will take a turn for the worse quickly. This is very frustrating because this team has so much potential and raw talent, players just need to be played in the right positions.
You rightly said it correctly. The defeat of Chelsea was caused by the wrong formation that Pochettion used to play against Nottingham Forest. A home team should have more of an attacking force at the front against their opponent.
By right, Chelsea shouldn't lose this kind of game at home to a lesser-known team when the league games haven't gone very far. Losing three points in today's match does not speak well of Pochettino's managing Chelsea as a coach. He needs to place all Chelsea players in the proper positions on the field so they can perform as expected rather than struggling to score and give their best effort.