Head coach Valerien Ismael criticised West Brom's defending despite them regaining top spot in the Sky Bet Championship with a 1-1 draw against Millwall at The Hawthorns.
Kyle Bartley's 49th-minute header was cancelled out by on-loan Arsenal defender Daniel Ballard's aerial effort six minutes later to end Albion's four-match winning league run on a day when previous leaders Fulham lost 1-0 at Blackpool.
Karlan Grant missed a penalty for the Baggies but they were undone Ballard's header, and poor back passes by Semi Ajayi enabled Jed Wallace and Sheyi Ojo to have one-on-one chances that were foiled by goalkeeper Sam Johnstone.
"We made too many mistakes in our backline and were punished for it," said Ismael. "We need to have another desire and another attitude to be better than this.
"We have to be more ruthless in our backline – that was our strength in the first five games so we need to get back to that and dominate our opponent.
"We will learn from this and adjust but we need to put more energy to address that intensity."
Ismael added: "We need to learn from this and adapt, from when the opponent does not want to take any risk and wants to take the intensity out of the game.
"The game plan from the opponent was another way we have to deal with and Millwall made it difficult for us.
"Nevertheless, we had the possibility to score from the penalty and after that we came back very strongly, scoring the goal just after half-time.
"But the big difference was we made a mistake with the back pass to the goalkeeper and that made them dangerous and gave the opponent the feeling they could get back into the game."
Albion led when Bartley rose above Jake Cooper to glance a header beyond Bialkowski from Conor Townsend's cross after Grant's knockback.
Millwall drew level when Jed Wallace crossed deep from the right and Ballard's far-post header somehow crept under the crossbar.
The home side had a let-off when Ajayi's mis-hit backpass presented Ojo with a one-on-one, but Johnstone blocked to his right.
Millwall manager Gary Rowett felt the penalty should not have stood, so thought justice was done by Bartosz Bialkowski's save from Grant.
"I didn't think it was a penalty – I didn't think Maikel Kieftenbeld moved his foot (to trip Grady Diangana) but Bartosz got us off the hook with a great save," said Rowett.
"If it's a clear penalty, then fine – but I think, if we'd have had VAR, I don't think it would have stood."
Rowett was however delighted with how Millwall's game plan worked as they stifled the hosts.
"West Brom are very good at playing pressure football against you and a lot of teams struggle to deal with it," he added.
"Every game we have watched, teams have struggled to deal with that intensity.
"I thought we managed that really well and dealt with the game.
"Overall, we created quite a few chances and possibly more and the better chances.
"We conceded a poor goal but we always looked threatening. Sheyi Ojo and Jed Wallace looked a major threat on the counter attack and we realty should have scored another goal in that period."
