Mata joined the Red Sox's player pool Tuesday, Alex Speier of The Boston Globe reports.
Mata will give Boston extra pitching depth this season, though he's unlikely to play a major role. The 21-year-old is yet to reach Triple-A and struggled to a 5.03 ERA in 11 starts for Double-A Portland last season, though that figure came with a strong 3.27 xFIP, as his 25.2 percent strikeout rate and 52.1 percent groundball rate offset a poor 10.3 percent walk rate.