The Baggies lost 2-1 on Saturday.
"The effort was definitely there and we had a real go but didn't create too many clear-cut chances," said midfielder Morrison.
"We'd fought our way back into the game, only to suffer a real sucker punch with that great strike from Sissoko.
"They say that after scoring the next five minutes are crucial but we slacked off, backed off and he put the ball in the top corner."
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