Mejia, 24, has a 5.06 ERA and a 1.58 WHIP in seven starts in 37 1/3 innings this season. He has 38 strikeouts (good!) and 20 walks (not good!). Mejia has allowed 16 runs and put 29 men on base in his last three starts and 14 2/3, prompting the move into the bullpen. He pitched in relief back in 2010.
Montero, 23, has a 3.67 ERA and a 1.15 WHIP in eight starts and 41 2/3 innings at Triple-A Las Vegas this season. Baseball America ranked him as the team's third best prospect before the season, saying "he could profile as a good No. 3 starter" in their 2014 Prospect Handbook.
The Mets will play the Yankees on Wednesday as the Subway Series shifts from Yankee Stadium to Citi Field. They're throwing Montero right into the fire.