Second-half goals from Morgan Amalfitano and Saido Berahino helped West Brom claim their first victory at Old Trafford in 34 years, despite Wayne Rooney having brought the hosts level with half an hour to play.
The result left United in the bottom half of the Premier League, with only two wins from their first six games.
But while Moyes admitted his concern at that opening to the season, he insisted he will put things right.
"I'm concerned after today but only because we didn't play well. But there's a lot of games to go and I'll try to put it right in the games to come," he said.
"There was (a lack of intensity) but I couldn't have said that about the game in midweek (a win against Liverpool). I thought that was the opposite from what we had in midweek.
"But I thought today we missed that real spark, something just to go on and make the difference and make it happen.
"And I think if we could have got a goal then it might have changed but West Brom deserved it. There was no question West Brom deserved their victory.
"(It was a) poor result and a poor performance," Moyes continued. "We never really got going. We had a lot of the ball in the first half but never made enough chances and when we did make some from it we didn't finish them.
"They always looked a threat on the break. We wanted try and keep the momentum going. I thought we opened up well in the game but it didn't lead to anything and obviously the performance went on to be a poor one."
Moyes made seven changes to his team from last weekend's 4-1 defeat to Manchester City, but he was adamant that he is not struggling to find his best starting XI.
"I don't think there is one best team, I think you've got lots of teams here at Manchester United," he added. "I think you play all the players and I did that again today.
"We changed the team in midweek, we played the bulk of them again today to give them a chance to make sure that it wasn't just one game and not getting another one - we wanted to give them a chance to do it again.
"I'll get to know them a lot better, the more I see them the more I'll get to know them."
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