Jose Mourinho loses his second match on the trot as the Swiss side grew in the second half to beat the Premier League outfit after Oscar broke the deadline just before half-time.
Six years to the day after a 1-1 draw with Rosenborg that represented the final game of Mourinho's first spell in charge at Stamford Bridge, Chelsea were stunned by second-half goals from Mohamed Salah and Marco Streller.
Oscar fired the hosts ahead in the 45th minute in the Group E clash with his sixth goal in seven Champions League outings and the Brazilian also hit the bar prior to Salah’s classy left-footed leveller.
That goal prompted Chelsea to throw men forward, yet it was Basel - always dangerous on the break - who struck again when Streller beat Petr Cech with a near-post header from a corner.
VIEW FROM STAMFORD BRIDGE
By Greg Stobart
Jose Mourinho would not have expected to hear his Chelsea players booed off the pitch in just his third home game in charge since his return to manage the club.
But the Portuguese can have no complaints following a dreadful performance that lacked any spark save for a couple of moments of brilliance from Oscar, who opened the scoring with a fine strike just before the break.
Chelsea have now lost two in a row in all competitions and have failed to win any of their last four fixtures, with owner Roman Abramovich looking glum-faced to say the least as he watched on at full-time.
Basel were stoic in defence and brilliant on the counter-attack. In Mohamed Salah they had the best player on the pitch and the winger equalised to finish a great team move in the 71st minute.
Nine minutes from the end, Basel captain Marco Streller headed home from a corner to scenes of bedlam on the away bench on an historic night for the Swiss club. It was no less than they deserved.Willian was handed his Chelsea debut and Marco van Ginkel a first start as Mourinho made five changes from the side beaten 1-0 by Everton at the weekend.
Basel recalled a host of players after fielding a weakened side for their Schweizer Pokal victory against Munsingen.
Mourinho's men unsurprisingly started the game on the front foot, albeit without creating any clear-cut opportunities.
There was a scare for Basel goalkeeper Yann Sommer in the 15th minute when he fumbled a corner from the left, but a free-kick was swiftly awarded in his favour.
The pacey Salah - a scorer at Stamford Bridge in last season's Europa League semi-final between the two sides - represented the main threat when Basel launched occasional attacks.
After Eden Hazard had spurned a rare clear sighting of goal for Chelsea following good work from Samuel Eto'o, Salah also proved wasteful when cutting in from the right and curling a shot over the bar.
Despite dominating possession and the corner count, the hosts remained frustrated as half-time approached.
However, Chelsea were lifted on the stroke of the interval as Oscar popped up with a timely goal.
David Luiz set the attack in motion by stepping into midfield and Oscar latched on to a delicate throughball from Frank Lampard to find the bottom-left corner with a first-time shot on the turn.
Streller was close to connecting with a left-wing cross early in the second half before Oscar struck the bar at the other end with a tremendous effort from just outside the area.
Hazard failed to hit the target once again either side of Branislav Ivanovic twice threatening from set-pieces.
Chelsea were then punished for not extending their lead when a cross from the left broke via Streller to Salah, who emulated Oscar by only requiring one touch to find the net.
Although Chelsea poured forward in search of a late winner, it was Basel who finished the stronger and their encouraging play gained reward as Streller shocked the home crowd
