Jimmy Shan said the goals his side conceded were the biggest disappointment of the 3-1 defeat at Derby County on Sunday.
The win sealed Derby's play-off place, while the Baggies will also face an extended season as they face local rivals Aston Villa having finished fourth.
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But Shan felt his side weren't good enough after falling behind in the second-half at Pride Park.
He said: "The biggest thing for me was the nature of the goals we conceded.
"Also, the work ethic and levels of application we showed after going 3-1 down were unacceptable.
"It's possible the play-offs have been a bit of distraction for us given we've known we'll be competing in them for a while now, but there is no excuse now and we have to prepare properly."
Shan is well aware of the threat Aston Villa will pose when the sides meet next Saturday and then the following Tuesday.
He said: "They are a good side but the standard of this division has meant that every game has been tough in a highly competitive league.
"Our momentum has taken a knock but the key thing will be to turn the mistakes and lessons learned from today into a positive."