James Shan claims Albion must do much better if they are to make it Wembley after defeat in the final game of the season.
Albion were again below par against a Derby side that needed a victory to guarantee their play off place.
The host took the lead when Martyn Waghorn was given the freedom of Albion's area to head home.
The Baggies then wasted two glorious chances when the poor Jacob Murphy blasted wide and then Dwight Gayle missed from six yards minutes later.
Albion did finally find an equaliser after half time when Stefan Johansen curled home.
But back came Derby with two quick fire goals from Mason Bennett and Harry Wilson.
Hal Robson-Kanu was then sent off in injury time for violent conduct meaning he'll miss Albion's play-off games.
"After they scored the third, our level of application and work ethic was not acceptable," Shan said.
"However we dress up what's happened today, it's not the business end.
"There were some honest assessments from the players in there. There's no excuse now.
"We're going to have to play a lot better than that if we're going to have some success against Aston Villa."