Jubilant Legia Warsaw fans stripped to the waist in the away end at Stamford Bridge on Thursday during a shock 2-1 win over Chelsea that laid bare the hosts' lack of intensity in the second leg of their Conference League tie.
The London club advanced to the semifinals of the third-tier European competition 4-2 on aggregate after winning 3-0 in Poland but Chelsea captain Reece James said they were well short of their best in the second leg.
Just over 1 000 tickets were allocated to Legia fans due to safety concerns and the majority of them had whipped off their tops when Tomas Pekhart fired them ahead from the penalty spot after 10 minutes.
Marc Cucurella levelled before the break but Steve Kapuadi restored the visitors' lead in the 53rd minute as the away end erupted in a sea of bare chests and swirling red smoke.
"Maybe we disrespected the competition today. But this is football. If you don't prepare right, you will pay," James said.
Chelsea manager Enzo Maresca made seven changes to the starting line-up that drew 2-2 with Ipswich in the Premier League on Sunday and said their comfortable advantage from the first leg was perhaps behind their lacklustre performance.
"Probably, after the first leg we tried to manage a little bit the result, rotate players, try to avoid injury, because we have an important game on Sunday."
Chelsea travel to Fulham in the league at the weekend and are set for a trip to Sweden's Djurgarden in the first leg of their semifinal on May 1.