Chelsea boss Antonio Conte did a post-mortem after Tuesday's 3-0 defeat to Roma in the Champions League.
Stephan El Shaarawy scored twice in the first half before Diego Perotti added a third as the Blues surrendered top spot in Group C to the Italians.
"When you concede three goals you must be worried because it means something doesn't work," Conte told BT Sport.
"It's very difficult now to tell but for sure we have to analyse the game. I think we needed to work on the tactical aspect. For sure, in this season, we are working less on this aspect and we are paying in this situation.
"I think we have to look at ourselves, our performance and to understand that tonight was a bad night for us. I hope, just one night.
"I actually believe we played a good match in the first half, but were 2-0 down. We could have scored after 30 seconds and then suffered on the break. We created many chances, were not clinical and ruthless enough, then suffered from their second shot on target.
"What I'm most disappointed about was the performance in the second half when Roma had more hunger, more desire, more fight to beat us and clearly deserved this win."