Mata (elbow) began a rehab assignment at Single-A Salem on Saturday, Alex Speier of The Boston Globe reports.
Mata was pitching in extended spring training last month, but he returned to minor-league game action Saturday and allowed an unearned run on no hits and two walks while striking out two in two innings. He generated 10 swings and misses during the outing and is expected to continue his rehab assignment at High-A Greenville this week.