Due to last night's rain out, the Mets are playing a traditional doubleheader today in Pittsburgh, and for the second game, they will have access to an extra bench player, called up specifically for the game, as part of the '26th Man Rule.' Eric Campbell is playing that role today for the Mets, as he will be activated before the nightcap, giving the Mets a little more flexibility on their already-strained roster.
The 26th Man Rule, established in the 2011 Collective Bargaining Agreement, allows a team to add an extra player to the roster for both games of a doubleheader scheduled 48 hours in advance, or in the second game of a doubleheader scheduled with less than two days' notice. Due to this game being announced last night, Campbell will only be available in the second game. You can read more on the rule here.
The Mets will always be tied to this rule, as former Met Jeremy Hefner was the first player to ever gain 26th-man status, when he relieved Miguel Batista in an April 23, 2012 game against the Giants, which was game one of a doubleheader.
Due to the partially torn thumb ligament for Juan Lagares, it might have been more advantageous to call up an outfielder for this position, but the only non-active outfielder on the Mets' 40-man roster is Brandon Nimmo, and he does not appear ready for a call up. It also seems silly to make a move on the 40-man roster just for this one game, so perhaps Campbell's two appearances in left field this year will become three before night's end. Ty Kelly has also gotten reps in the outfield in his minor league career and could be an option in the corners if need be.