Williamson will be part of the Mariners' 60-man training roster.

Williamson is one of the top sleepers in Seattle's farm system. The 6-foot-6 southpaw would have likely been assigned to High-A in a normal season, and while he has a chance to be a No. 2 or No. 3 starter long term, the Mariners probably won't rush his development to the point that he would pitch in MLB games this season. He had a 2.35 ERA, 0.91 WHIP and 25:5 K:BB in 15.1 innings with short-season Everett last season after getting drafted out of TCU with the No. 59 overall pick.