In my view, there were no good solutions here. Whatever MLB and the teams came up with would have been an issue for one side or the other. Matters were further complicated by ESPN taking the Mets-Phillies game for Sunday night -- a decision made within the last few weeks as per ESPN's ability to flex games.
Had that last part not happened, MLB could have had the teams play Monday-Wednesday with the third game in the afternoon. Or a worst case scenario being Monday with a Tuesday doubleheader. They also could have relocated the series or part of it to Arlington since the Rangers weren't home this week (I presume the stadium was available). I'm sure they could have found another location if need be.
Now the Mets are faced with potentially having to travel from Milwaukee to Atlanta on Sunday night, play an afternoon doubleheader on Monday, then travel Monday night to Milwaukee, San Diego or Los Angeles to play on Tuesday.
Evan Roberts on WFAN also noted that if MLB doesn't push the NL wild card series back a day, both NY and Atlanta will likely not have a regular starting pitcher available on normal rest to start a Tuesday game one.