Arsene Wenger labelled Arsenal's defeat to West Brom a "nightmare" as they missed the chance to go top.
The Gunners went down 2-1 at The Hawthorns to lie behind Leicester, Manchester United and Manchester City in the Barclays Premier League.
The Baggies clinched their first home win over the Gunners in 10 years after Santi Cazorla slipped and missed a late penalty.
James Morrison's volley and a Mikel Arteta own goal gave the hosts - who only managed one shot on target - victory after Olivier Giroud opened the scoring.
Wenger said: "It was a very bad afternoon for us. We had 70 per cent possession, we scored an own goal, we missed a penalty, we gave away a goal from a set-piece, that is perfect to make the afternoon a nightmare.
"We missed open-goal chances. I believe West Brom did fight with 100 per cent commitment as well and you have to give them credit for that.
"At 1-0 we had just to make sure they did not come back with an easy goal and that's where we were guilty, because apart from that I cannot fault the team who gave what they could until the last minute. W e produced enough to win the game."
The French national anthem La Marseillaise was played before the game in tribute to the victims of the Paris terrorist attacks last week and Wenger believes his French players, Giroud and Laurent Koscielny, were focused.
He said: "Once you are in the competition you are in the competition to be professional. How much did it affect them? I don't know, I can only speak for myself.
"When you are on the pitch, you are on the pitch. I would not explain our defeat by that."
Arteta and Francis Coquelin limped off to add to Arsenal's injury woes ahead of their must-win Champions League game with Dinamo Zagreb on Tuesday.
Mesut Ozil also hit a post as the visitors dominated, but failed to break down the Baggies in the second half.
This was just Albion's second home league win of the season, but it lifted them to 12th and eight points above the bottom three.
Boss Tony Pulis said: "There's been games here this year where we've been disappointed because we've lost. I thought Arsenal were very good, particularly second half when they ramped it up.
"Sanchez was just wonderful - his movement, his play.You've got Ozil and Cazorla who have wonderful talent. The lads deserve it for the effort and commitment, but we've played better and done better and not got the results."
But Pulis admitted he feared Cazorla was going to get a second chance to take his penalty after Mark Clattenburg's whistle created some confusion, but it emerged the referee had blown to rule Cazorla had kicked the ball twice as he fell.
"I thought he was going to take it again to be perfectly honest. It's a double-touch apparently and that's what he's given," added Pulis.
"He actually kicked it twice. It wasn't as soft as some of the penalties given against us this year."