Mata will make his debut in Tuesday's Premier League clash with Cardiff at Old Trafford knowing his club have slipped six points adrift of a Champions League qualifying slot after an awful campaign.
The 3-1 defeat at Chelsea on January 19 was their seventh defeat of a season that has also seen them exit the FA Cup at the third round stage and lose a Capital One Cup semi-final to relegation threatened Sunderland.
It has led to a growing view that United are floundering at the highest level and are set for a period in the wilderness.
Moyes is not prepared to countenance that and while he accepts further big-money additions might have to wait until the summer, there will be some changes.
"I am not going to accept it," said Moyes.
"I am disappointed we are not in a much stronger position. I am disappointed with how we have played.
"I, ultimately, take the wrap for that.
"But what I will do is make it right. I am going to get better players in.
"Unfortunately, I don't think there will be more in January. But it will change."
The exit of Anderson on loan to Fiorentina, plus the impending departures of Wilfried Zaha and Fabio further confirm Moyes' intentions.
It is impossible to see Rio Ferdinand's contract being renewed given his recent lack of games, while the futures of skipper Nemanja Vidic and full-back Patrice Evra are also in the balance.
With Alexander Buttner, Nani, Ashley Young and Javier Hernandez among those who also have questions marks hanging over them, the Moyes revolution is under way.
"I have given every player the opportunity to play and show what they can do," he said.
"We have had a pretty good chance to have a look at it.
"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."
The strength of Moyes' words should meet with the approval of Wayne Rooney, who has indicated the potential for signing a contract extension but - like Mata presumably - will want clear signs United are intent on returning to their former status.
Rooney said much the same when he handed in his transfer request in 2010, although in that instance, his fears were dismissed by Sir Alex Ferguson, who told the England man team-building plans were none of his business.
Moyes was not prepared to discuss Rooney in front of the cameras, stating simply: "I've left that to Ed (Woodward) and Wayne and his advisors. we'll give you an update at the right time."
For now he is just happy to bring in some much-needed quality.
"I got wind a month or two ago it could be a possibility. We went after it right away but I never believed we would pull it off," said Moyes.
"Congratulations to the club and Ed Woodward for getting the deal done.
"There will be more players like Juan (Mata) in the future, I have no doubt about that but my gut feeling is that it will be the summer."
Moyes accepts they will probably not be heading to Old Trafford prior to Friday night's transfer deadline, which merely highlights the failure to strengthen last summer.
"The summer transfer window wasn't a good one for us," said the Scot.
"I was only just in the job. We were getting to know everyone and we had a new chief executive as well."