Tony Pulis said Albion had a bad day at the office after Crystal Palace's 2-0 win at The Hawthorns.
Albion barely got out of first gear and struggled to create any clear cut chances in a poor first half.
It didn't get any better after the break and Wilfried Zaha gave the visitors the lead ten minutes into the second half.
Andros Townsend scored a second for the visitors in injury time with a fine solo effort.
"No excuses today. We weren't at it right from the start. Congratulations to Palace," said Pulis
"I'm not going to criticise the players because they have been fantastic this year. We've had an off day and I hope that is all it is.
"Why was it an off day? If I knew I'd be a genius. We've worked hard and trained hard this week. We were tepid right from the very beginning."
Why were Albion so poor against Palace? Have your say at Baggies Banter.