The reigning Premier League champions have put up a limp defence of their title and sit in seventh place in the table, 14 points behind leaders Arsenal and scrapping just to qualify for the Champions League.
While the Scot accepts responsibility for United's recent problems, he says he will no longer tolerate sub-par displays and has promised fans to expect changes.
“I am not going to accept it. I am disappointed we are not in a much stronger position. I am disappointed with how we have played. I, ultimately, take the rap for that,” said Moyes.
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The former Everton boss had given the entire squad a chance to impress upon his arrival at Old Trafford, but, having been in the job for over half a season now, is ready to axe those players no longer performing.
“"I have given every player the opportunity to play and show what they can do. We have had a pretty good chance to have a look at it,” continued Moyes.
“The players have to perform. If they want to be here and making sure they have a jersey, they have to show it in the games. Undoubtedly this club wants to be at the top. We want to be challenging. At this moment in time we have failed to do that. We want to do something about it.”
Despite United's struggles, Moyes is confident the capture of Juan Mata proves the club are still a force to be reckoned with, though he admits he would like to have added a few more players to accompany the Spaniard.
“The message this sends, as I said in the summer, is that we're going for all the best players in the world,” added the 50-year-old. “There will be more players like Juan in the future, no doubt about that.
“I've got a big job to do here and a couple of new signings isn't all that is needed. I think there's a bigger job to be done but you have to start somewhere.
“If you asked me what I needed most, I would have said a central midfielder. I could do with a left-back, too, but sometimes you don't always get the pieces of the jigsaw at the time. It's all part of a rebuilding process.”