Slaten was unavailable for Tuesday's game against the Brewers due to oblique soreness, Chris Cotillo of MassLive.com reports.
The team has labeled Slaten as day-to-day, so his injury doesn't seem to be too severe. The 28-year-old is off to a strong start this season after striking out five batters over 3.1 shutout innings, picking up holds in two of his four appearances.