Graham Potter says Joao Felix's red card was the turning point in Chelsea's 2-1 defeat to Fulham, but the under-fire Blues boss understands why the debutant was dismissed.
Chelsea had levelled Thursday's contest at Craven Cottage when Joao Felix was sent off by referee David Coote for a studs-up challenge on Kenny Tete.
Carlos Vinicius went on to head home a winner for Fulham as they beat their west London rivals for only the fourth time in 53 top-flight matches, and the first time since 2006.
Potter was left to rue a sloppy challenge from Joao Felix, who was thrust straight into the starting XI after arriving on a short-term loan deal from Atletico Madrid on Wednesday.
He is the first Chelsea player to be sent off on his Premier League debut, and the first for any club since Federico Fazio for Tottenham against Manchester City in October 2014.
"It was a forwards' tackle. There was no malice in it but I understand why it was a red," Potter told BT Sport of the Portugal international's lunge on Tete.
"It is another blow for us; the hits keep coming at the moment. [Joao Felix] was really good. You could see his quality in the game so it is doubly disappointing for us."
Willian had given Fulham the lead against his former side Chelsea, but the visitors responded through Kalidou Koulibaly's close-range finish after Mason Mount had hit the post.
Carlos Vinicius' first goal for Fulham prolongs 10th-placed Chelsea's terrible run of form, which now stands at seven defeats in 10 matches in all competitions.
"I think we controlled the game early, won the ball back high, created some opportunities, maybe missed the last pass. We made a couple of mistakes for their goal," Potter said.
"Second-half I think we responded well and then the red card changes the game. It makes it a little more difficult for us.
"But I thought the application when we went down to 10 was really good. I'm disappointed with the [winning] goal because I think we could do better and that is what cost us."
Chelsea's return of 25 points from their first 18 Premier League matches is their fewest at this stage since the 2015-16 campaign (18 points), when going on to finish 10th.
Further questions will inevitably be asked of Potter, who only succeeded Thomas Tuchel in September, but the Englishman has called on his squad to stick together.
"We have got to keep working, there is no other solutions," he said. "You have to keep working stay together and try to get the three points."
Former Chelsea winger Willian did not celebrate his first-half opener against old suitors Chelsea, whom he won five major honours with earlier in his career.
"I had an amazing seven years at Chelsea," said Willian, who is the 26th player to score against Chelsea in the Premier League after previously playing for them.
"I have a big respect for the fans and the club. I play for Fulham now and have to do my best. It was a special night.
"We deserved the win. We know how difficult it is to play against Chelsea. They are in a difficult moment but we are a really good team. We deserved it and we have to carry on."
A fourth top-flight win in a row for Fulham – the first time they have achieved that since April 1966 (a run of five) – moves them within four points of the Champions League spots.