3) Paul Blackburn
Unlike the previous two players mentioned, Paul Blackburn's performance isn't the reason for his reduced role. After being acquired at last year's trade deadline, the thought was that he could be a reliable back-end of the rotation arm. However, he made only five starts for the Mets before being placed on the injured list and has not pitched since August 28th in San Diego.
While he's still rehabbing from offseason surgery to repair a cerebrospinal fluid leak, he might not have an MLB job to come back to. With the rotation pitching to the best ERA in all of baseball, it's hard to imagine taking anyone out. Plus, Sean Manaea and Frankie Montas would be ahead of him in potentially taking any spot in the rotation. Even if the Mets go to a six-man rotation, it's unlikely Blackburn is the one they call on.
The other role Blackburn could've filled is the long man in the bullpen. Have him come in to cover multiple innings if the starter can not go deep enough in the game, to try and save bullpen arms. However, the Mets currently have three relievers who have demonstrated their ability to pitch multiple innings. Making a Blackburn addition pretty redundant. It seems that Blackburn has no clear path to the major league roster, except in the event of a potential injury.