Weaver, who signed a major-league deal with the Mariners on Tuesday, will work in a long-relief role out of the bullpen, Ryan Divish of The Seattle Times reports.
With Emerson Hancock (shoulder) done for the year, Weaver would likely be the next man up if the Mariners need a spot starter, but there aren't any plans to go to a six-man rotation. He had a 5.57 ERA, 1.67 WHIP and 21 strikeouts in 21 innings over his last five outings for the Reds before they released him last week.