Slaten (oblique) allowed one hit and struck out two over a scoreless inning for Double-A Portland on Sunday.
Slaten opened the rehab assignment by serving as a game opener for the Sea Dogs. The right-hander's next outing will be Tuesday or Wednesday at Triple-A Worcester, per Chris Cotillo of MassLive.com. It's uncertain if Slaten will require more rehab outings beyond what's coming up in the next two days. He last pitched for Boston on April 4.