"There will be nothing happening," the United manager told reporters. "I said at the start that we wouldn't do much business in January."
Spaniard Mata made his debut in United's 2-0 win over Cardiff City on Tuesday and Moyes has been delighted with how the 25-year-old has adjusted to life at Old Trafford.
"Juan (Mata) has been everything I thought," Moyes said of United's record signing.
"He's a terrific footballer, a good lad to work with and he's settled in quickly.
"He played well in the game (against Cardiff), which was important because we wanted him to make a difference straight away.
"I'm wary that we need to give him time to settle in and not expect too much too soon, but I was impressed with his first game."
There may not be any more incomings at Old Trafford, but Moyes has let 21-year-old winger Wilfried Zaha join Cardiff City on loan until the end of the season.
"We felt Cardiff was the right choice for him," Moyes added.
"Hopefully he will go and get those games, show what he can do in the Premier League and be ready to come back to us at the end of the season."
(Reporting By Michael Hann, editing by Ed Osmond)
