The former Chelsea midfielder scored a brace during last weekend's 4-0 win over Newcastle United.
Charlton, who spent 17 years at the club, told the team's website: "I like Juan Mata a lot. I must admit I admired him the very first time I saw him play on television, but I did not think we would have the opportunity to sign him.
"He is a fantastic player already but could go on to become a great one. I'm sure that's his aim.
"In the coming months and years, he will get the opportunity to score and make more goals because he is simply first class. You can already see how much he loves being here and he is a very nice man to go with it."
Mata made the move to Old Trafford during the January transfer window.