As he did in the vital victory over title rivals Manchester City at the Etihad Stadium eight days ago, Branislav Ivanovic gave Chelsea a first-half lead on Tuesday.
But, after Samuel Eto'o spurned a pair of chances to give Chelsea breathing space, substitute Victor Anichebe netted a fine header from Saido Berahino's 87th-minute cross to give West Brom a 1-1 draw and just reward for mounting late pressure.
Victories for City and Arsenal on Wednesday night would drop Chelsea to third position in the table and Mourinho felt events at The Hawthorns backed up his recent claims that his team are not yet the finished article.
"I think a bit of everything," he said when contemplating two important points dropped.
"The attacking players stopped playing and made the opponent feel more comfortable, to come forward with no fear.
"At the same time, my defensive line started playing too deep. We gave them the ball and we just waited for them to put the ball into the box and to try and fight against it.
"But that is not our game; that is not the way we feel comfortable.
"I think maybe also, a little bit of a lack of personality because we must have the personality to get out and not accept their pressure and playing for so long in the last third.
"The game could end 1-0 but I think their goal gives some truth to their reaction."
He added: "A ready team, a complete team kills this game. Two-zero, goodbye. We didn't."
Shortly before the equaliser, Ivanovic was booked for a heated exchange with both the match officials and his own goalkeeper Petr Cech, but Mourinho refused to condemn the player and praised a display of passion and emotion
"I don't know what happened, but what happened is good," he said.
"I like the players to have emotion. If somebody made a mistake I like the players to speak between them(selves).
"Someone told me then that Ivo was complaining with one of the officials for some reason but I couldn't see (anything) wrong."
