David Moyes admitted West Ham suffered a bad day at the office after they were beaten 3-2 by Southampton.
A late header from Jan Bednarek condemned the Hammers to defeat after they had twice hauled themselves level in the Premier League clash at the London Stadium.
Southampton took the lead after only eight minutes when Mohamed Elyounoussi finished from the edge of the penalty area.
Substitute Michail Antonio equalised before James Ward-Prowse put Saints back in front with a penalty after Craig Dawson brought down Armando Broja.
Said Benrahma swept the Hammers level again only for Bednarek to nod in a Ward-Prowse free-kick and leave them empty-handed.
“I didn’t think it was anything like us. I’m hoping I can make it an off-day for us. It was not the West Ham I’ve been with in the last two years,” said Moyes.
“You can have off-days. We’re not any different. Today was a poor one. I thought we’d done enough to get ourselves back in the game and turn a bad day into a good day.
“In the first half we were really poor. We didn’t have enough energy. We couldn’t build any momentum. To be honest, I could have changed four or five at half-time.
“I felt there would only be one winner at that point. Even at 1-1, I thought we’d go on to win the game. Or at least not lose it.”
The Hammers kicked off in fifth place but are now set to lose further ground on the top four.
“We’ve got to stay in a good, strong position, and stay on the tails of the teams above us if we can, and keep moving on,” added Moyes.
“The first half was flat. It lacked energy from what we would normally be like. At least they improved in the second half, the players.
A first win in seven matches gave Saints boss Ralph Hasenhuttl a welcome festive boost.
He said: “Every threat they had on the pitch, we had the right answer. It was an important win and three points, and fantastic that we can come here and win.
“We played a bit different today. We can be more resistant against this team and scoring three goals here is fantastic, and an important step for the team.
“We scored goals at good moments. Tactically it was a very brave performance.”