Chelsea manager Jose Mourinho says his side "couldn't cope" with West Brom after seeing them drop two points in the race for the Premier League title.
Branislav Ivanovic looked to have sent the Blues four points clear at the top of the table after putting his side ahead at West Bromwich Albion.
But Baggies substitute Victor Anichebe scored an 87th-minute equaliser to earn a 1-1 draw for the home side.
"West Brom had a reaction and we couldn't cope," said Mourinho.
"I thought even if I sent on some attacking players, it wouldn't help because we were under pressure. It was impossible for my defenders to get out of the first third of the pitch.
"They started putting long balls into the box, with some free kicks, corners and rebounds, and fighting for those."
Asked why his team sat too deep, Mourinho replied: "A little bit of a lack of personality.
"We must have the personality to get out, to not accept the pressure.
"The game could perfectly have ended 1-0, but we have to give some truth to their reaction."
Instead of cementing their position at the top, Chelsea will be replaced at the summit if either Arsenal or Manchester City win on Wednesday.
The Gunners are at home to Manchester United, while City host Sunderland.
Chelsea were dominant for long spells but their failure to add to Ivanovic's second goal in three games proved costly as West Brom climbed out of the relegation zone with a battling draw.
The Blues had recorded clean sheets in each of their previous four league and cup games.
But Mourinho blamed his defence - as well as a lack of cutting edge - for the failure to hang on for three points at The Hawthorns and accused his players of "lacking personality" .
"For 60 minutes there was only one team, and that team couldn't kill the game, especially in the first 15 minutes of the second half," he added.
"That is football. The team stayed back, we were too deep."