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 Mourinho’s 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 away skipper Marco Streller won the Group E opener for the visitors with a late near-post header.
Chelsea manager Jose Mourinho: “I’m not in shock. Sometimes you win sometimes you lose. The reality is that sometimes you don’t win. I’m not happy of course. We have taken a step back in qualification but we have to assume responsibility. We have five games left to finish in the top two.
“Basel played very well. We had a lot of the ball but we couldn’t create many chances. Basel equalised at our best moment of the game. After that we were punished defensively. We are strong at defending corners. Emotionally this is not a mature team and when you are in a difficult position it was not enough.
“I rotate my team because I have to. I kept the structure of the team and the players must fight for position. It was not individuals who lost the match.”