Jose Mourinho's first Champions League tie since returning to Chelsea ended in disappointment as Swiss champions Basel earned a shock win at Stamford Bridge.
Oscar put the home side ahead with an angled first-time shot into the bottom corner at the end of the first half.
Basel's Mohamed Salah levelled when he fired across Blues keeper Petr Cech.
And the home crowd was left stunned as skipper Marco Streller won the Group E opener for the visitors with a late near-post header.
But the Blues, who have won the Champions League and Europa League in the past two seasons, expected a change in fortunes as they returned to European action.
Instead Basel consigned Chelsea to their first Champions League group stage defeat at Stamford Bridge since October 2003.
The shock loss came exactly six years after Mourinho took charge of the final match of his first Blues spell, a 1-1 draw against Norwegian side Rosenborg.
The Portuguese, who returned from Real Madrid in the summer, has played down his youthful squad's chances of winning the Champions League - describing them as "young eggs who need nurturing".
"When Mourinho was winning trophies at Chelsea last time around, he knew how to get results. But I think the game's changed. Players like Hazard and Oscar need to be released to go and play, go and hurt teams.
"I've watched them this season and they seem a little rigid. It's like they're in a straitjacket. You don't see Ronaldo or Messi tracking back.
"I can't believe Mata can't get into the team because he's a man who can open doors."
But the team he selected on Wednesday could hardly be described as inexperienced. Including £30m debutant Willian, Chelsea dominated possession in the opening hour, but only had Oscar's strike to show for it.
The home side forced six corners in a dominant first-half display, without being able to force Basel goalkeeper Yann Sommer into a serious save.
Sommer did find himself in trouble when he fumbled an Oscar set-piece under pressure from Gary Cahill and home debutant Samuel Eto'o in his six-yard box, only to be rescued by referee Daniele Orsato who generously awarded a foul against the home players.
Chelsea were finally rewarded just before the interval when Oscar fired a first-time shot into the bottom corner after latching on to a perfectly weighted throughball by Frank Lampard.
The Brazil midfielder also hit the crossbar after the break as Chelsea searched for the security of a second goal. But it did not arrive and Basel, dangerous on the counter-attack throughout, punished their hosts.
Salah, who scored on the same ground when the teams met in last season's Europa League semi-final, had repeatedly tested Chelsea left-back Ashley Cole and it was no surprise that he was the man to level the scores.
Basel's victory was the first win by a Swiss side in 20 competitive visits to England. The previous winners were FC Luzern who triumphed 2-0 at Tottenham in the 1995 UEFA Intertoto Cup.
Neat build-up from the visitors ended in Streller picking out the unmarked Egypt international, who curled a low left-foot shot beyond Cech.
Mourinho introduced Juan Mata and Demba Ba as the Blues pressed for a late winner, but they only looked more vulnerable as Basel grew in confidence.
The home side scrambled clear after midfielder Fabian Schar burst through the middle, before team-mate Marcelo Diaz had a shot blocked.
But the Swiss side continued to press and grabbed the decisive goal when Streller escaped two Chelsea markers to head home with eight minutes to go.
Basel and German side Schalke, who earned a comfortable 3-0 win against Romanian visitors Steaua Bucharest, top Group E after the opening round of matches.
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Live Text Commentary
Full Time The referee blows the whistle to end the game.90:00 +2:11
The ball is crossed by Juan Mata. Kay Voser makes a clearance.90:00 +1:06
Samuel Eto'o takes a shot. Yann Sommer safely holds on.89:09
Emboaba Oscar takes a shot. Yann Sommer safely holds on.88:34
The ball is crossed by Eden Hazard. Clearance made by Marco Streller.87:31 Substitution
Substitution Mohamed Salah goes off and Taulant Xhaka comes on.86:57
Branislav Ivanovic produces a cross. Fabian Schar manages to make a clearance.86:46
Branislav Ivanovic fouled by Mohamed Salah, the ref awards a free kick. Free kick taken by Branislav Ivanovic.83:16
Marcelo Diaz takes a shot. Blocked by Gary Cahill.82:59 Substitution
Substitution Arlind Ajeti on for Valentin Stocker.81:04
The assist for the goal came from Kay Voser.81:04 Goal scored
Goal! - Marco Streller - Chelsea 1 - 2 FC Basel Marco Streller scores with a header. Chelsea 1-2 Basle.80:48
Corner from the left by-line taken by Kay Voser.80:21
Shot by Marco Streller went wide of the left-hand upright.75:15 Substitution
Substitution Demba Ba joins the action as a substitute, replacing Frank Lampard.74:56 Substitution
Substitution (Chelsea) makes a substitution, with Mikel coming on for Marco van Ginkel.72:07
Unfair challenge on Samuel Eto'o by Marcelo Diaz results in a free kick. Juan Mata takes the free kick.70:33
Marco Streller provided the assist for the goal.70:33 Goal scored
Goal! - Mohamed Salah - Chelsea 1 - 1 FC Basel Mohamed Salah scores with a left foot finish. Chelsea 1-1 Basle.69:30
Marco Streller is caught offside. Petr Cech takes the free kick.
Free kick awarded for an unfair challenge on Marco Streller by David Luiz. Marco Streller restarts play with the free kick.66:24 Substitution
Substitution (Chelsea) makes a substitution, with Juan Mata coming on for da Silva Willian.63:50 Booking
The ball is delivered by Frank Lampard. Header at goal by Branislav Ivanovic misses to the right. Short corner worked by Frank Lampard. Frank Lampard decides to take a short corner. Emboaba Oscar takes a shot. Blocked by Marcelo Diaz.63:46 Substitution
Substitution Matias Delgado replaces Giovanni Sio.63:46
Marcelo Diaz challenges da Silva Willian unfairly and gives away a free kick. Marcelo Diaz is booked.63:20
Valentin Stocker gives away a free kick for an unfair challenge on Petr Cech. Free kick taken by Petr Cech.61:23
Eden Hazard takes a shot.57:48
Corner taken by Frank Lampard from the right by-line. Branislav Ivanovic takes a shot. Yann Sommer safely holds on.57:20
Shot by Emboaba Oscar missed to the right of the target.55:32
Emboaba Oscar takes a shot.53:33
Corner taken by Valentin Stocker.53:19
Behrang Safari crosses the ball. Clearance made by Ashley Cole.51:57
Giovanni Sio gives away a free kick for an unfair challenge on Emboaba Oscar. Gary Cahill takes the free kick.51:40
Foul by Valentin Stocker on Emboaba Oscar, free kick awarded. Free kick taken by Frank Lampard.49:56
Da Silva Willian crosses the ball. Marcelo Diaz manages to make a clearance.48:58
Unfair challenge on Marcelo Diaz by Samuel Eto'o results in a free kick. Yann Sommer restarts play with the free kick.48:39
Free kick awarded for a foul by Giovanni Sio on Gary Cahill. Gary Cahill takes the free kick.47:35
The referee blows for offside. David Luiz takes the free kick.46:02
Behrang Safari challenges da Silva Willian unfairly and gives away a free kick. Da Silva Willian takes the free kick.45:01
The game restarts for the second half.45:00 +0:17 Half time
Half Time The half-time whistle blows.44:08
Assist on the goal came from Frank Lampard.44:08 Goal scored
Goal! - Emboaba Oscar - Chelsea 1 - 0 FC Basel Emboaba Oscar scores with a right foot finish. Chelsea 1-0 Basle.43:04
Gary Cahill takes the chance to get an effort at goal. Ivan Ivanov gets a block in.42:38
Marcelo Diaz gives away a free kick for an unfair challenge on Eden Hazard. Frank Lampard takes the free kick.41:32 Booking
Giovanni Sio fouled by Marco van Ginkel, the ref awards a free kick. Marco van Ginkel goes into the referee's book for unsporting behaviour. Free kick crossed by Fabian Frei. Petr Cech catches the ball.39:55
Frank Lampard concedes a free kick for a foul on Marcelo Diaz. Free kick taken by Ivan Ivanov.38:22
Samuel Eto'o challenges Fabian Frei unfairly and gives away a free kick. Free kick taken by Kay Voser.36:11
The offside flag is raised against da Silva Willian. Yann Sommer takes the free kick.34:36
A cross is delivered by Emboaba Oscar. Yann Sommer safely holds on.33:44
Corner taken by Frank Lampard from the right by-line. Giovanni Sio manages to make a clearance.33:30
Branislav Ivanovic sends in a cross. Clearance made by Ivan Ivanov.30:59
Giovanni Sio takes a shot. Petr Cech catches the ball.30:30
Marco van Ginkel concedes a free kick for a foul on Giovanni Sio. Free kick taken by Fabian Frei.28:59
Shot by Mohamed Salah cleared the net.28:16
Unfair challenge on Samuel Eto'o by Mohamed Salah results in a free kick. Free kick taken by Frank Lampard.25:45
Eden Hazard takes a shot.23:57
Shot by Frank Lampard went wide of the right-hand post.21:09
The assistant referee flags for offside against Ashley Cole. Free kick taken by Yann Sommer.18:17
Marco Streller fouled by da Silva Willian, the ref awards a free kick. Free kick crossed by Marco Streller. Frank Lampard makes a clearance.17:34
Foul by David Luiz on Marco Streller, free kick awarded. Free kick taken by Marco Streller.15:53
Centre by Mohamed Salah. Gary Cahill manages to make a clearance.14:45
Corner taken by Emboaba Oscar. Free kick awarded for a foul by Samuel Eto'o on Yann Sommer. Free kick taken by Yann Sommer.14:12
Foul by Behrang Safari on da Silva Willian, free kick awarded. Branislav Ivanovic restarts play with the free kick.12:51
Short corner worked by Eden Hazard.12:29
Fabian Frei gives away a free kick for an unfair challenge on Eden Hazard. Eden Hazard takes the free kick.7:45
Corner taken short by da Silva Willian. Da Silva Willian sends in a cross. Clearance made by Fabian Schar.7:34
The ball is delivered by da Silva Willian. Blocked by Ivan Ivanov.6:27
Ashley Cole fouled by Mohamed Salah, the ref awards a free kick. Petr Cech restarts play with the free kick.5:47
Foul by Marco van Ginkel on Mohamed Salah, free kick awarded. Fabian Frei restarts play with the free kick.5:12
Corner taken by Frank Lampard. Clearance by Ivan Ivanov.5:03
The ball is sent over by da Silva Willian. Behrang Safari gets a block in.2:16
Corner taken by Frank Lampard. Clearance by Giovanni Sio.2:02
The ball is swung over by Emboaba Oscar. Ivan Ivanov gets a block in.0:41
Free kick awarded for an unfair challenge on Giovanni Sio by Branislav Ivanovic. Behrang Safari takes the free kick.0:00
The match has kicked off.
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