Gsellman suffered a left oblique injury during Tuesday's loss to the Orioles, Deesha Thosar of the New York Daily News reports.
The 27-year-old worked in long relief Tuesday and surrendered six runs on six hits and two walks over 3.2 innings while throwing a season-high 76 pitches before sustaining the injury. The team has yet to provide an official timeline for his return, but given the nature of the injury, Gsellman seems likely to require a stint on the injured list.