Trade for Austin Slater
Jerry Dipoto has come out recently and said that in one of the corner outfield spots there will most likely be a platoon. On the left side of that, expect to see a lot of Dominic Canzone, who I think could really break out and replace Kelenic's production. We could also see some of Cade Marlowe, maybe even Taylor Trammel or Zach Deloach.
The Mariners will need to work to find the right-handed side of this platoon as they currently don't have any other right-handed outfield options other than Dylan Moore, and as I have stated, I think he is a strong trade candidate. In searching for the right-handed side of the platoon, the Mariners should turn to the San Francisco Giants, who now have a gluttony of outfielders after signing Jung Hoo Lee. They should target Austin Slater, a guy who has absolutely mashed against left-handed pitchers.
Like I said, Slater has absolutely dominated lefties. In his career, 787 plate appearances, he has slashed .285/.374/.463 which is good for a 131 wRC+. He has impressive CTZ rates as well, a 10.7% walk rate and a 22.7% strikeout rate. Slater is also a slightly below average centerfielder and can probably give you about average defense in left field, he just doesn't have a great arm, but he won't kill you out there. He also has an impressive baseball savant page.
With an average exit velocity of 91.1 mph, that is over 2.5 mph harder than the MLB average. He is also really good at finding the barrel, with an 11.8% barrel % which is almost double the MLB average of 6.9%. Finally, like Max Kepler, Austin Slater is a 1-year rental, but is fairly cheap at $4 million. For a platoon bat, one that is one of the best bats against lefties, that is a pretty solid deal, one that could prove very valuable for the Mariners.
I don't think Slater would be very expensive to acquire for the Mariners as he is a 1-year rental and is a platoon bat, but the Giants are a team that doesn't necessarily need to move him. They would move him if they got more value out of moving him than keeping him, say for a reliever. The Giants may also be looking to get more athletic, so someone like Cade Marlowe or Dylan Moore could make a lot of sense for them as well.
After these moves you would still have a 1-5 rotation of Castillo, Kirby, Gilbert, Miller and Woo. And you lineup would go something like JP, Julio, Soler, Kepler, Cal, Ty France, Canzone/Slater, Urias, Rojas. That is a lot more respectable lineup and one that I would put money on having a higher floor and being more reliable than the lineup they rolled out last year.