I asked a question about this on the Soxprospects forum. They deduced that Mata was very likely the guy the Red Sox turned down in the Sale trade. Here is a response I got from Chris Hatfield:
"So, a) His scouting report has been updated. Ian saw him at Instructs and came back very impressed. He put him in his top 30 right away and convinced us to push him up past where he sat after we all voted. Then, after the trades, Dave Dombrowski was on WEEI talking about them and the process. He referred to a player in the process that another team asked for who was 1. Venezuelan, 2. had played in the DSL, and 3. had pitched at Fall Instructs, and the Red Sox had refused to trade him. Presuming that these were the negotiations for either the Thornburg or Sale trade, that means that they at least liked this player more than Victor Diaz (21 before the trade) or Josh Pennington (27 before the trade). The only two players who fit the description Dombrowski gave were Mata and Ritzi Mendoza. Now, Mendoza also had a very good year in the DSL... but he was 21 and repeating the league for the fourth time. So there's a 99% chance they refused to include Mata in a deal in which they did trade a player we had ranked higher.
Between Ian's report on him (which I'll let you all read), the numbers and other reports we've read, and the Dombrowski thing, it seemed pretty clear we had to move him into the Top 20."
It seems to me very little is still known from him but he had good numbers in the DSL, he impressed at fall instructs, and we can deduce from the Sox front office that they really really like him.