Mata, he said, was the start of a revolution at Old Trafford. And while there would be more arrivals of similar quality to the Spanish midfielder, there would also be casualties.
He told the Manchester Evening News it was all in the name of "change."
I have a clear idea of what I think I need to do. We will rebuild and form another strong Manchester United. There will be changes at the club.
The changes Moyes has in mind might have to wait until the summer. And that gives some of United's players a stay of execution and four months to win over their new manager.
Mata's arrival, and the emergence of Adnan Januzaj, raise doubts about the future of Shinji Kagawa.
The Japanese midfielder is more comfortable playing centrally, but with Wayne Rooney, Robin van Persie and Mata in the team, it's difficult to see where he's going to fit in.
Even without Mata, Kagawa only made 15 starts under Moyes, most of them out wide. And when Mata came off late in the second half against Cardiff, it was Januzaj who took his place.
Nani signed a new contract in the summer, but while he's been injured, he's fallen behind Antonio Valencia and Ashley Young. Tom Cleverley, meanwhile, should get another season to show his development is going in the right direction.
United's defence is also set for a major shake-up.
Rio Ferdinand, Nemanja Vidic and Patrice Evra, the backbone of the back four for the last seven years, are out of contract at the end of the season and it's possible they will all leave.
It's likely Moyes will chase a new left-back in the summer, but Jonny Evans, Chris Smalling and Phil Jones have between now and the end of the season to show they deserve a regular place at centre-half in United's new-look team.
Moyes also has cause to be disappointed with Rafael's performances this season, but if the Brazilian can rediscover last season's form, he won't be allowed to leave.
Javier Hernandez's situation is different.
His ability to come off the bench and change a game makes him a valuable member of the squad, but a desire for more regular first-team football could see him agitate for a move away.
However, he's under contract until 2016 and if Moyes doesn't want to let him leave, he won't.
Anderson and Fabio have already gone, and there are a few others at United who will be fearing for their futures.
But Young has shown it's never too late to make a good impression.
After a poor game against Manchester City in September, he started just one of the next 16 games. But after starting against Cardiff, creating one goal and scoring the other, he's back in Moyes' good books.
It's proof that it's not too late for some players to earn places in United's squad for next season, but they'll have to win over Moyes first.