The visitors came into the game on the back of three successive top-flight victories, but they were beaten by Brown Ideye's 19th-minute goal.
They failed to truly trouble a stubborn West Brom defence thereafter, as Tony Pulis got one over on his former employers.
"We started the game slowly," Hughes said. "We talked at length about not conceding the first goal, and we didn't really as them any questions whatsoever.
"West Brom were quite comfortable defending what we were able to produce.
"There was a slight reaction after half-time but we didn't really maintain it and in the end West Brom were able to defend and win the game quite comfortably.
"That's the disappointment for us because we've been playing well up to today and I didn't see that performance coming. We know we're better than that.
"Too many underperformed. We can't afford to carry anybody. Six or seven under-performed and we can't win Premier League games if we have that happening.
"It's hopefully a blip. We've got nine games now and I don't intend for us to put in a performance like that again from now until the end of the season.
"We'll use this as motivation for the games we have left."
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