United came from behind to defeat Paul Lambert's men at Old Trafford on Saturday, with Mata drawing the foul from Leandro Bacuna for Wayne Rooney to put the hosts ahead from the penalty spot, before the ex-Chelsea man slotted home his side's third goal.
It was the Spaniard's first for United after agreeing a club-record £37.1 million deal in January.
Moyes said Mata's goal has been a long time coming, and paid tribute to the 25-year-old for making a positive contribution on the team.
"He's deserved it, his play has been very good, he's made a difference to the team he's helped the team," he said.
"Goals were one of the reasons we bought him, he's always scored goals, he's got a very good goalscoring record.
"I think he was probably feeling it a little bit, he missed a chance in midweek, missed two good chances against West Ham, so I think there have been signs, the opportunities have been coming for him to score goals.
"But whenever he's played he's made a difference and done very well for us."
