Chelsea 0 Nottingham Forest 1 - Highlights
Nottingham Forest came away from Stamford Bridge to spoil the party for Chelsea
Chelsea's spending on deadline day took their total splashed on new players in just over a year under a consortium fronted by Todd Boehly to more than £1 billion ($1.3 billion).
Moises Caicedo's move from Brighton, that could rise to £115 million, broke the British transfer record last month.
But the Ecuador international was at fault for the only goal at Stamford Bridge to leave Mauricio Pochettino's men embarrassed.
Anthony Elanga slotted home three minutes into the second-half after Caicedo gave the ball away inside his own half.
New £40 million signing Cole Palmer was thrown on to try and turn the game around, but Chelsea's lack of a natural goalscorer, despite their huge outlay, was again exposed.
Pochettino has won just one of his first four games in charge to leave Chelsea already eight points adrift of City.
"We were not clinical in both areas," said Pochettino. "We made a mistake and then we created opportunities, but if you don't score it is difficult to win games.