Mayer was held out of Double-A Portland's lineup Tuesday and Wednesday due to pectoral soreness, Chris Hatfield of SoxProspects.com reports.
The Red Sox are reportedly viewing the shortstop prospect's injury as a day-to-day concern, so the expectation is that he'll avoid a stint on Portland's injured list. Mayer missed time earlier this month while battling back discomfort but has performed well in the six games that he's played so far in July, slashing .292/.370/.500 with a 3:7 BB:K.