John Terry's Chelsea return went awry as the European champions crashed to their first major defeat of the season, losing 2-1 at Shakhtar Donetsk.
The Blues were ruthlessly punished by Alex Teixeira and Fernandinho goals at the Donbass Arena, despite Oscar's late consolation, as Shakhtar deservedly took charge of Champions League Group E in the first of what could be a crucial double-header.
Chelsea were spared dropping out of the qualification places by Juventus' shock draw at Nordsjaelland, but they will need to do much better at Stamford Bridge in two weeks to avoid doing so.
Terry's first appearance since accepting his racism ban got off to the worst possible start as he unwittingly set up Shakhtar's opener inside three minutes. But he was arguably the least culpable Chelsea player on a night which cost the Barclays Premier League leaders dearly and a risible performance from Fernando Torres scuppered their hopes of a comeback.
The defeat was their first since their UEFA Super Cup thrashing by Atletico Madrid and the performance was almost as alarming at times as they were carved open by an outstanding Shakhtar side, who went into the game having not lost for exactly 11 months while winning their last 21 league games.
And it was a nightmare start for the defender as the home side scored from their first attack after two minutes and 53 seconds, Luiz Adriano's shot hitting Terry's elbow and falling perfectly for Teixeira to slot past a helpless Petr Cech.
It immediately got worse for the visitors when Frank Lampard went down injured, forcing an early end to his record 100th European appearance for the club and the introduction of Eden Hazard.
But Chelsea were looking capable of equalising, but after the break Hazard produced a horrendous gaffe to gift Shakhtar their second goal in the 52nd minute. He was pickpocketed in the centre-circle by Fernandinho, who fed Adriano and both raced goalwards for the former to slot past Cech.
Hazard tried to make amends with a low drive tipped wide by Andriy Pyatov, who then palmed a Ramires blockbuster behind for a corner.
Hazard was then thwarted one-on-one with Pyatov, before Oscar set up a tense finish two minutes from time when he swept home Branislav Ivanovic's cross.