Thibaut Courtois called on Chelsea to rediscover their fighting spirit after Roma stormed to a 3-0 win over the Premier League champions at the Stadio Olimpico.
Antonio Conte's side ceded top spot in Champions League Group C and were left with an uphill task when Stephan El Shaarawy clattered home a stunning opener inside the first minute.
The former AC Milan youngster doubled Roma's advantage before the break and Diego Perotti took advantage of a flagging Chelsea to drive home a superb third after the hour.
Chelsea's defence, so often impregnable last season, shambled towards full time and were indebted to goalkeeper Courtois for not being on the end of an even heavier loss.
"We knew Roma would start with intensity and we had to equal it but after 50 seconds we were down 1-0. A second ball and El Shaarawy scores a screamer," Courtois told BT Sport, before explaining he thought communication issues were at play as Antonio Rudiger's woeful error allowed the Germany international's former club to double their lead.
"The second goal is a bit unfortunate," he said. "I saw El Shaarawy going towards the back of Toni and I shouted 'away'.
"He let it go. Maybe he heard something else. It was a loud stadium.
"I thought we played well in the first half but in the second half we seemed to drop our fighting spirit and I don't know what happened."
Manchester United travel to Stamford Bridge on Sunday and Courtois knows improvements must be immediate.
"It's hard to say what the problem is," he said. "Last season I had sometimes nothing to do but this season, every game is a hard game for the team - the defence and everyone.
"We have to find that balance again and keep that mentality of not conceding goals and keeping clean sheets.
"We have to improve on Sunday. Today was not a good image of ourselves and we have to react."