The Mets-Cardinals game scheduled to take place at Citi Field this evening was rained out and postponed. It will be played on Tuesday at 4:10 PM as a straight doubleheader. If you had tickets to the game, the Mets have details about their rainout policy on their website.
The Mets don't have an off day this week, which means they'll probably need to give someone a spot over the weekend. With a rotation that already includes Logan Verrett, who's filling in for Matt Harvey, that's not ideal. Seth Lugo, who has thrown 7.1 innings out of the Mets' bullpen, spent some time in the Triple-A Las Vegas 51s' starting rotation to begin the season but hasn't made a start since early June.
So the most likely options seem to be the left-handed Sean Gilmartin or the right-handed Gabriel Ynoa and Robert Gsellman. Gilmartin acquitted himself nicely out of the Mets' bullpen last year, while Ynoa and Gsellman haven't yet pitched in the big leagues.