After his Brazilian countryman Oscar fired Chelsea ahead on 45 minutes, Mohamed Salah and Marco Streller scored in the second half to hand the Swiss club the win.
The loss was Jose Mourinho’s second on the trot, and the crowd reaction at Stamford Bridge was understandably negative.
Willian, who was substituted off for Juan Mata on 67 minutes, admits his first match for Chelsea was disappointing.
“It’s good to debut [but] it would be even better if it was winning,” the 25-year-old said. “Now we have to raise our heads and try to win [the next game].”
The Blues travel to Romania on October 1 for their next Champions League clash against Steaua Bucharest.
“Debuts always have a bit of anxiety but I think I could show some of my football” Willian continued.
“I know that I have to try get better every day. I can’t stop, I always have to try and keep a good pace and that’s what I’ll do. I have to show every game, every training session, and then [Mourinho] will be able to believe in me.”