With star forward Alexis Sanchez left at home, Arsenal squandered a lead by conceding two goals in five minutes to lose 2-1 at Bournemouth in the Premier League on Sunday.
Callum Wilson equalized in the 70th minute for Bournemouth before setting up Jordon Ibe for the winner in the 74th minute, intensifying Arsenal's problems in what could be developing into another crisis under Arsene Wenger.
Hector Bellerin opened the scoring in the 52nd for Arsenal, which is in sixth place and has dropped eight points off the Champions League qualification positions as a result of Liverpool's win over Manchester City later Sunday.
Wenger said Sanchez was omitted because the Chile international was "being vague" while City and Manchester United fight for his services. It seems inevitable that Sanchez will depart this month, leaving the team without its game-changer and top scorer from last season.
"Half in, half out," Wenger said of Sanchez's current position at Arsenal. "You never know this kind of situation during the transfer period."
Mesut Ozil was also missing because of injury and Arsenal's lineup looked a shadow of the past teams under Wenger.
"We were 1-0 up and suddenly we lost two goals and we don't know where they came from," Wenger said. "It is very frustrating. Overall we have to look at it in a very objective and harsh way - we made mistakes we should not have made.
"The team that fights against relegation will fight and you cannot afford any concentration mistake."
It was a first Premier League win over Arsenal for Bournemouth, and a first victory against a top-six side this season that lifts Eddie Howe's team into 13th place.
"In this division, if you can get two or three wins your whole outlook can change," Howe said. "Then if you get two or three defeats it all changes. We have just tried to be very stable.
"That was a famous win for the club."