Mikel Arteta admitted Arsenal's shock 2-0 defeat at Bournemouth was an "accident waiting to happen" as William Saliba's red card condemned them to a first Premier League loss since April.
Arteta's title-chasing side played for an hour with 10 men after French defender Saliba was dismissed for a professional foul on Evanilson.
Referee Rob Jones initially showed Saliba a yellow card but the punishment was upgraded to a red after VAR intervened.
It was the third time Arsenal had a player sent off in the league this season after Declan Rice saw red against Brighton and Leandro Trossard was dismissed at Manchester City.
Arteta conceded the disciplinary problems were making it harder to win, with Arsenal taking just two points from the three games played with 10 men.
"It's very difficult to win in the Premier League with 10 men for 60 minutes. It's just an accident waiting to happen. We have had to go through it in three games and that cost us the game," he said.
"We had the big chance and the moment in the game to score and have a different result and then we conceded a goal.
"We try to stick to the rules. The three red cards have been different ones and three very different outcomes."
Beaten in the league for the first time since Aston Villa's 2-0 victory at the Emirates Stadium last season, Arsenal's loss cost them the chance to go above Liverpool at the top of the table.
'FANTASTIC EFFORT'
Arteta acknowledged his third-placed team were below their best for long periods at the Vitality Stadium.
"The team tried, the effort with 10 men was fantastic but today it was not enough to win the game," he said.
"It took us a while to understand what we had do in the first 15 minutes, then we grabbed the game but after 30 minutes the game changed."
Arsenal midfielder Rice was even more honest in his assessment of the lethargic performance.
Trossard's wayward pass was the trigger for Saliba's dismissal as the centre-back was forced to make a panicked tug on Evanilson.
It was a careless mistake by Trossard and Rice said Arsenal must stamp out that kind of blunder if they want to mount another title challenge.
"We've kicked ourselves in the foot three times in eight games and we got away with it at home to Brighton and away at Manchester City. Bournemouth kept probing and made it 2-0," Rice said.
"I'm proud of the players for fighting, even with 10 men, but the naivety, we need to stop making mistakes because you want 11 players for 90 minutes and that's what wins you football matches.
"We can't make silly mistakes. You need all your best players on the pitch at all times. The belief is so high and we will stick together."