The Hammers boss said that he was happy with how his team had played until falling behind, but was frustrated with the lack of creativity after the break.
"We did not have enough cutting edge," the 59-year-old told BT Sport.
"That was the problem for us and, having had no problems in the first half, we were looking to try and probe and get on target.
"But we gave away the most ridiculous goal, from our point of view, that I've seen for a very long time."
The West London outfit remain three points clear of the Premier League drop zone.