Liam Rosenior admitted Chelsea paid the price for costly mistakes after Arsenal took advantage of his side's blunders to win 3-2 in the League Cup semifinal first leg on Wednesday.
Rosenior's team face a tough task to set up a final against either Manchester City or Newcastle following their error-strewn display in their new manager's first home match.
Chelsea were guilty of sloppy marking for Ben White's early headed opener before goalkeeper Robert Sanchez gifted striker Viktor Gyokeres Arsenal's second goal after halftime.
Alejandro Garnacho got one back for Chelsea but Martin Zubimendi then netted for Arsenal after more lacklustre defending from Rosenior's men.
Substitute Garnacho's second goal gave Chelsea a glimmer of hope heading into the second leg at the Emirates Stadium in February.
"Disappointed to concede from a corner. Disappointed with the third goal as well because we were right back in the game and we were on top at that moment," Rosenior said.
"We switched off from a restart from a central free-kick but I can't fault the players.
"We need to make sure we perform well individually and we don't concede as many goals."
Rosenior was without a host of key players, including Cole Palmer, Reece James and Liam Delap, due to injuries and illness.
'IT'S ANOTHER STEP'
In his second game since replacing Enzo Maresca as Blues boss, the 41-year-old took heart from the way Chelsea kept fighting to find a way back into the tie.
"We've had illness in the squad, we've picked up a few knocks this week but what the squad has shown is that they are willing to run and fight for each other," he said.
Rosenior, who oversaw a 5-1 FA Cup third-round win at Charlton in his debut last weekend, refused to condemn Sanchez for the latest in a long line of shaky performances.
"Rob's a very good goalkeeper. He made an outstanding save at 3-1 to keep us in the tie, so for me load of things to improve but the overall attitude of the team I liked," Rosenior said.
"Hopefully, we get a few bodies back for Brentford on Saturday."
Arsenal are now unbeaten in 10 games in all competitions as they moved a step closer to their first silverware since the 2020 FA Cup.
The Gunners had lost their previous four semifinals across a variety of competitions, including the League Cup last year.
Mikel Arteta was impressed with Arsenal's ability to subdue Chelsea for long periods, but he was left to rue their failure to kill off their London rivals.
"I have to praise the players for the performance against a really good opponents. It's a really tough place to come. That's why I really value what the team has done again," Arteta said.
"We had two massive chances to score the fourth one and the result would have been very different. At that moment they created a chance and scored a goal. So it is a very different feeling. It's game on."
As well as leading the Premier League, Arsenal are also still chasing Champions League and FA Cup glory.
But after so many last-four failures in the recent past, Arteta won't take anything for granted.
"It's another step. It's just halftime. We know the big fight we are going to have at the Emirates in a few weeks because they are a top side," he said.
"What we're doing every three days is impressive."