Mariners activate Criswell’s return as Fenway roster plans shift for Montes’ arrival

The Mariners walked into Fenway Park on Monday with September roster expansion on the mind, and two bullpen-and-prospects storylines were already taking shape. According to the team’s official announcement, Seattle is welcoming Cooper Criswell back after a time away from game action, and the organization is also setting up to add slugging prospect Lazaro Montes within about 24 hours.

Criswell’s return is framed as a meaningful bullpen addition for the Mariners late in the season. In September, when roster sizes expand and managing pitchers becomes a bigger day-to-day challenge, getting a familiar arm back can affect everything from leverage matchups to how long relievers can stay available. The team’s release characterizes the move as a welcomed return to the group, suggesting the Mariners view his presence as more than just depth.

Montes, meanwhile, represents the other half of Seattle’s roster plan for this phase of the year: adding a prospect bat with power. The announcement indicates he is expected to join the Mariners’ club Tuesday, aligning with the idea that September is as much about sharpening depth and evaluation as it is about immediate wins. The Mariners described him as a slugger, placing emphasis on impact potential as his call-up becomes imminent.

The timing matters because the Mariners’ first stop on this stretch is Boston, a park where late-inning runs can swing quickly for teams that rely on power and run prevention. While Monday’s decision points are about personnel, Tuesday’s addition of Montes suggests Seattle could be adjusting its lineup toward a heavier offensive look than it might otherwise carry with a standard 26-man group.

Roster expansion in September does not happen in a vacuum. Even when teams add players they expect to contribute, every move also creates downstream decisions, including how starters and bullpen arms will be used over consecutive games and travel. With Criswell returning and Montes on a near-term timetable, Seattle’s staff and everyday lineup could both look different by the middle of the week than they did at the start of the series trip.

For Mariners fans, the most immediate thing to watch is how the coaching staff integrates Criswell back into the bullpen’s usage patterns. The second is Montes’ role once he arrives, including whether he is used as an early-impact power option or asked to work into the lineup. If Seattle is using the September period to both win games and test what it can count on, these two names provide a clear window into that balancing act.

Both additions are being handled as part of the organization’s late-season planning rather than as one-off moves. With Fenway as the setting and a Tuesday arrival date already established for Montes, the Mariners appear ready to make targeted adjustments that strengthen their pitching in the near term while adding a power bat quickly enough to matter during the trip.