Mayer (back) is starting at shortstop and batting second in Friday's game for Double-A New Hampshire, Alex Speier of The Boston Globe reports.
Mayer returns to the lineup Friday after missing four games due to back discomfort. The 21-year-old prospect is slashing .307/.371/.479 with 25 doubles, 13 stolen bases, seven home runs and 36 RBI over 299 plate appearances in Double-A this season.