Arsenal manager Arsene Wenger has described his side's 2-1 loss to West Brom on Saturday as "a nightmare" after they slipped to fourth in the Premier League.
The Gunners looked on track for a routine win at the Hawthorns as Olivier Giroud headed home the opener after 30 minutes, but the Baggies responded just moments later through James Morrison.
Mikel Arteta then bundled James McClean's delivery into the back of his own net to give Albion the lead, and although Arsenal were handed a lifeline with an 82nd-minute penalty, Santi Cazorla slipped at the vital moment to balloon his effort over the crossbar.
The defeat was even more meaningful after Manchester City, league leaders on Saturday morning, crashed to a 4-1 loss at home against Liverpool in the day's final match, meaning a win would have left Arsenal top of the Premier League, instead of just inside the top four.
And, speaking after the game, a clearly dejected Wenger said: "It was a bad afternoon, we missed a penalty, we gave a goal away on set-piece, that is perfect to make your afternoon a nightmare and we missed open goal chances.
"I believe that West Brom, they were strong in the fight and they had one 100 per cent commitment as well, and you have to give them credit for that.
"But overall I felt it dropped a huge blow when we scored and we just had to make sure they didn't come back with a easy goal and that is where we were disappointed today.
"But I cannot be mad at the team because they gave what they could until the last minute but we lost a bit of focus at 1-0 – at 35 minutes we were 1-0 up and then at 45 we were 2-1 down. Our defending on that level was very, very poor."