The best choice for the Mariners would be to trade one of their veteran outfielders, whether it is Domingo Santana, Mallex Smith or Mitch Haniger. Personally I would begin with trading Smith. His defense wasn’t good in 2019 and his hitting was not as expected either. If he stays with the Mariners, it should not be as the centerfielder.
Santana and Haniger would get the best value in return. With Santana, the Mariners could get something good in return during this off-season while Haniger might be better to trade during the season before the trading deadline since he was out most of this past season.