SEATTLE -- Jerry Dipoto couldn't help but chuckle when Jarred Kelenic went deep in his first at-bat at the Triple-A level.

It was yet more evidence Kelenic was ready for the big leagues.

"I laughed out loud when he had a homer in his first Triple-A at-bat off a lefty, because that's about as appropriate to how Jared Kelenic accepts challenges as you can imagine," Dipoto said Thursday. "And I'm sure he's going to be up for this next one, which is facing the best pitchers in the world night after night."

Kelenic and right-handed pitcher Logan Gilbert are making their debuts for the Mariners on Thursday night as Seattle takes the next step in its rebuilding project. Kelenic is starting in left field and battling leadoff, while Gilbert will start on the mound as Seattle opens a four-game series with Cleveland.

"Players, sometimes they hit the ground running, and sometimes they don't. The major leagues are difficult," Dipoto said. "But there's never any more excitement than with the anticipation of the arrival of talented young players like these and we're excited to see what happens across the board. It's gonna be a fun time for us."

The additions Thursday are part of a major roster reshuffle for the Mariners. Coming up from Triple-A Tacoma along with Kelenic and Gilbert is right-hander Paul Sewald to bolster a taxed bullpen.

But Kelenic and Gilbert are the headliners. Kelenic has been regarded as one of the top prospects in the minors for several years and only reinforced his status with a torrid start at Triple-A, where he hit .370 with six runs, two homers, a double and five RBIs in just six games.

"He's knocked every challenge we've given him out of the park, really, and has done that since he's been a Mariner," Dipoto said.

Kelenic will primarily play in left field, but Dipoto expects him to move around the outfield on days Kyle Lewis or Mitch Haniger need a break from playing in the field.

Kelenic said some of his nerves were eased because Gilbert was debuting on the same night.

"To be able to start this journey in the big leagues with someone that you started the journey with in the minor leagues makes this transition a little bit easier," Kelenic said. "I was just talking with him in the clubhouse and as excited as I am to get going with my major league career, I'm just as excited for him."

Gilbert's arrival in Seattle was partly out of necessity. Dipoto said the team started targeting May for Gilbert's potential arrival late last season. But major injuries to Seattle's starting rotation may have prompted a slightly quicker decision on Gilbert, who made just one start in Triple-A.

With Seattle's plan to use a six-man rotation, Dipoto said the Mariners believe they'll be able to control Gilbert's innings and keep him around the target of 110 to 120 innings pitched this year.

"He's, I believe, as ready for this opportunity as a young pitcher can be," Dipoto said. "That doesn't mean it's going to be easy for him. It's quite a challenge in the big leagues. But he's so cerebral. He has been preparing for this start all of his life."

Manager Scott Servais said the goal with Gilbert would be to get about five innings. He also revealed Gilbert had shoulder trouble early in spring training that has limited his workload to date.

"He's completely healthy now. He's good to go and the goal tonight would be about 85 pitches," Servais said.

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. 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 for Tommy John surgery. Dipoto said it appears both players could be done for the year.

Seattle also designated for assignment outfielder Braden Bishop.