Kelenic, the No. 6 pick in the 2018 amateur draft, and Gilbert, the No. 14 pick in the 2018 draft, moved up from Triple-A Tacoma on Thursday ahead of their debuts that night against Cleveland. Gilbert will start on the mound, while Kelenic is expected to start in left field and bat leadoff as two of Seattle's prized prospects finally reach the majors.
Seattle also selected right-hander Paul Sewald to bolster a taxed bullpen.
To clear space on the roster, Seattle optioned relievers Aaron Fletcher and Wyatt Mills to Tacoma, along with outfielder Taylor Trammell, whose spot is being taken by Kelenic for now. Trammell made his major league debut on opening day, but is batting .157 in 27 games.
Manager Scott Servais said Trammell needs more playing time and expects he'll be back with the major league club at some point.
Seattle also transferred left-handed pitcher Nick Margevicius and right-hander Ljay Newsome to the 60-day injured list. Margevicius has been out since April 26 with shoulder issues. Newsome may be headed toward Tommy John surgery.
Seattle also designated for assignment outfielder Braden Bishop.
Gilbert is ranked as the No. 43 prospect in baseball, according to ESPN's Kiley McDaniel.
Gilbert made his Triple-A debut for the Tacoma Rainiers on Friday. He allowed one run on four hits with no walks and five strikeouts in five innings of work.
Gilbert, popped up in the Cape Cod League the summer before his draft year, working in the mid-90s with above-average stuff and starter command. During his sophomore season at Stetson, he was pitching deep into games, regularly well over 100 pitches, and his velocity was hovering in the low 90s with a less sharp breaking ball.
He profiles as a bulk-inning midrotation starter with solid average stuff and plus command, according to McDaniel.
The 18-18 Mariners, third in the AL West entering Wednesday, have had to lean heavily on their bullpen after injuries to Marco Gonzales (left forearm strain), James Paxton (season-ending Tommy John surgery) and Nick Margevicius (left shoulder inflammation). Seattle is down to four healthy members of its six-man rotation to start the season: Yusei Kikuchi, Justus Sheffield, Chris Flexen and Justin Dunn.