Backup Plan Two: Sign one big arm, trade for another, load up the bullpen
The Mets do need to add one starting pitcher of great significance. Luis Severino isn’t him. He is a wild card in this rotation. If the Mets even want to compete for a Wild Card spot, they'll need that big boost in the rotation. We can settle on one of those upper echelon guys. As a Yamamoto backup plan, they can add more around this addition.
If being competitive without spending too much money is the goal, pulling off a trade for a controllable starting pitcher is a way to ease any pain the misfire on Yamamoto could cause. Likely, the Mets will be in the market for a controllable starter rather than someone nearing free agency even if he is an immediate extension candidate.
Along with this, the Mets can load up the bullpen. The current status is about as dire as it gets. Starting pitching is the priority, but without a better relief corps, this team is doomed.
Adding two or three significant relievers to the bullpen will indirectly make up for not signing Yamamoto. An awesome bullpen won’t help them in the early innings. However, the abundance of lockdown options in the latter innings will help them in a different way.