The Baggies also missed a host of good goalscoring opportunities before bowing out of the competition by losing 2-0 at The Hawthorns.
"We got nothing because we made a really poor defensive mistake in the first half," the Baggies boss said after the game.
"That kind of quality is always going to find the spare man and that is what they did.
"The second goal and the manner in which it came about really took the wind out of our sails. You can't play like that and expect to get away with it.
"I think if we had scored first it might have made things different.
"But once you give them that first goal cushion their big hitters are just able to keep the ball with ease."