Darren Moore failed to criticise his players after Albion's winning streak at The Hawthorns came to a humbling end.
The Baggies were two down inside 15 minutes as sloppy defending allowed Jack Marriott and Tom Lawrence to put the visitors ahead.
It was a familiar story after the break when The Rams scored another two goals before Jay Rodriguez scored a late consolation goal for Albion.
"I thought it was a bizarre game tonight," the Baggies boss claimed.
"The goals came at crucial times - we had a couple of opportunities.
"The incident that changed it was the Dwight Gayle one. He gets clipped on the side of the box and picks up a slight knock so it was a double whammy. We should have got something from that.
"Even at 2-0 we had chances and were in the ascendency before half-time. It just seemed that everything Derby touched hit the back of the net tonight.
"Credit to them, they came here, set their stall and took their opportunities.
"For us it's about regrouping again. We've got a very important game here on Saturday.
"The physical demands of this league are huge.