Yeah if the Cardinals acquire another starter it will probably be via trade. The Mariners beat writer, Ryan Divish, said on MLB Network a couple of days ago that Mariners might be looking to move Luis Castillo because they need to purge more salary due to their regional tv network issues.
Castillo is still a high end pitcher as he finished 5th in the Cy Young voting in 2023. He is owed approximately $100M over the next four years which isn’t cheap but should make the asking price less painful than a deal for Cease. It might be something for Mo to consider although there wouldn’t be room for bullpen reinforcements unless Matz was traded to someone else.