Arsene Wenger refused to hide his anger at Arsenal's 3-1 defeat to Swansea as Olivier Giroud bade a likely farewell to the Gunners' fans.
Arsenal's one-goal lead was spectacularly overturned as Swansea escaped the bottom three in the Premier League for the first time in three months.
Pierre-Emerick Aubameyang is set to join Arsenal ahead of Wednesday night's transfer deadline, but this defeat leaves them eight points from fourth place after Liverpool's 3-0 victory at Huddersfield.
Giroud came on for the final 15 minutes at the Liberty Stadium, but the France striker's five-and-a-half-year stay at the Emirates Stadium is set to end on Wednesday with Chelsea his probable destination.
"We were absolutely guilty to concede the goal," Wenger said of Sam Clucas' equaliser just moments after Nacho Monreal had given Arsenal the lead.
"We have no rational explanation for the way we gave the goal away straight away."
And on Swansea's second goal -- which came from a poor Shkodran Mustafi back pass and a scuffed Petr Cech clearance straight to Jordan Ayew -- Wenger said: "I was angry because there was no need to let the ball go out in the throw in.
"When you have a throw in for you and you are nine vs. 10 on the pitch, why should we put a handicap like that on our own team? We can play the ball and attack straight away."
Aubameyang was at Arsenal's London Colney training ground on Tuesday afternoon as an expected club-record move inched nearer.
Giroud, meanwhile, is set to move across London to Stamford Bridge in a reported £18 million deal.
Swansea's win takes them out of the bottom three for the first time since Nov. 4, Clucas adding a double to Ayew's fifth goal in nine games.
Manager Carlos Carvalhal has taken 10 points from five league games since succeeding Paul Clement, and now has back-to-back wins against Liverpool and Arsenal.
"When I arrived at the club we were breathing like we were not well, but now after receiving some oxygen we come alive again!" Carvalhal said.
"Now we are breathing and we are not dead anymore.
"In this moment, you know that the word I kept hearing in my first press conference was the word 'miracle.'
"Now we don't achieve anything, but we are in a good way to achieve a good position.
"So far, the way the players are playing, the commitment that they show each other is really good.
"But at this moment we are still in a bad position."