Chelsea must rediscover their "hunger" after a 3-0 Champions League defeat by Roma, says manager Antonio Conte.
Stephan El Shaarawy scored twice as Roma moved above the Blues, who could have progressed to the knockout stage with a win, to the top of Group C.
Chelsea will go through if they beat Qarabag in their next group game.
"If you are a great team, you must have stability and consistency," said Conte. "In this moment we are struggling to have this balance."
The Italian guided the Blues to the Premier League title in his first season - with just five defeats in 38 games.
They go into Manchester United's visit on Sunday fourth in the table, having lost three of their opening 10 games.
"I don't know what will happen against Manchester United," said Conte. "They are a really good team.
"We must find the hunger that we showed last season and in this season sometimes. This season is up and down."
Chelsea's title success was built on a solid defence, but they struggled against their Serie A opponents.
El Shaarawy's first came after just 39 seconds, when the Blues could not clear a long ball, with his second following when Antonio Rudiger and Cesar Azpilicueta failed to deal with a pass forward.
The defence backed off Diego Perotti for the third and almost conceded a comical fourth when all three centre-backs ran to Edin Dzeko, leaving Perotti free in the centre to shoot over.
"The second half was a great disappointment. It was really bad for our team," said Conte.
"We have to have the strength to understand and to use this loss in the right way. We have to dig very deep if we want to be competitive and to fight for something important.
"This season will be very difficult if we don't understand quickly the right way that we have to go. My task is to try to find the best solution to try to avoid this type of game.
"We have to work together, we need to have the will to dig deep because if we think only because our name is Chelsea that the opponents have fear for the name Chelsea, this is not the right way.
"Last season we showed our great anger and will to do something of importance. Last season we won the league and did a miracle. But if you want to be competitive this season, we have to avoid these ups and downs. We must have stability and great desire to be protagonists in every game."
'I don't know what happened'
Keeper Thibaut Courtois told BT Sport: "I'm not sure how [El Shaarawy] scores that screamer but after 50 seconds you are a goal down.
"I shouted 'away' for the second goal but there was a lot of noise in the stadium and Cesar Azpilicueta let the ball go.
"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.
"It is hard to say what the problem is. This season, every game is a hard game.
"We have to find that balance again. And keeping that mentality of not conceding goals and keeping clean sheets."
Analysis - 'Whoever sold Matic should be sacked'
Former Manchester United defender Phil Neville at the Stadio Olimpico
What's gone wrong for Chelsea? One word. Matic. It's as simple as that. The minute they sold Nemanja Matic to Manchester United was a mistake.
When you had Matic sat in front of you, alongside N'Golo Kante [who missed Tuesday's game through injury], there was protection.
Whoever made that decision to sell him needs sacking. That is one of the poorest decisions I have ever seen in the Premier League.
If you ask any of those Chelsea players who they would want back, they would say Matic.
I wouldn't have let him out of the building. I would have chained him to the training ground walls.
They will need to take a long, hard look at themselves before the game this weekend. Chelsea have been well beaten.
Every time I have seen Antonio Conte this season he hasn't looked relaxed. He would not have wanted or expected to come back to his home country and be embarrassed like he has been.
I wonder whether he has to pull back that attacking football he likes to play to protect this back three.
The defenders were shocking. This is not the Chelsea side we saw last season. They looked all over the place.