PHOENIX - Bartolo Colon had been tossing a gem, in position for what would have been his ninth victory, the most in the major leagues.
The righthander operated with only a one-run cushion, which was all that a depleted Mets lineup could muster on his behalf Saturday night. Until the seventh, he had made it work. But with one transgression, the slim lead evaporated, and the Mets found themselves careening toward a 2-1 loss to the Diamondbacks.
After a night of pinpoint command, Colon left an 85- mph fastball belt high and over the heart of the plate. Welington Castillo, who entered play hitting .149 for his third big-league team in the last month, responded by smashing a two-run homer.
The Mets trailed by only one. But with a lineup besieged by injuries, even the most modest deficits have proved difficult to overcome.
They rallied in the ninth, putting the tying run in scoring position with the help of catcher's interference, but John Mayberry Jr. hit into a game-ending double play.
The Mets' razor-thin margin for error was further diminished without one of their hottest hitters in the lineup. Shortstop Wilmer Flores, who is hitting .300 during a seven-game hitting streak, was given the night off after complaining of general stiffness.
Juan Lagares hit his first homer in almost a month, skimming a drive off the top of the rightfield fence in the second for a 1-0 lead.
But for the second straight night, the Mets loaded the bases in a critical spot and came away with nothing, this time when Eric Campbell lifted a harmless fly ball to end the sixth.
Colon held the Diamondbacks to two runs in seven innings, a breakthrough after posting a 7.31 ERA in his previous five starts. He also collected his fourth hit, extending his unlikely hitting streak to three games. But all he got for his trouble was his fourth loss.
The Mets (30-27) dipped a half-game behind the Nationals for the NL East lead.