It only took until August, but Robert Gsellman might finally be rounding into form for the Mets.

The disappointing right-hander gave the team one of his strongest performances of the season Monday night, but was long departed from the scene by the conclusion of the Mets’ 3-2 loss to the Diamondbacks in 10 innings at Citi Field.

A.J. Pollock smashed a two-run homer against Erik Goeddel in the 10th to give the Diamondbacks the go-ahead runs on a night the Mets — who lost for the seventh time in eight games — went 2-for-12 with runners in scoring position.

Gsellman lasted 6 ¹/₃ innings and allowed one earned run on five hits and one walk with three strikeouts. He departed in a 1-0 hole, but the Mets tied the game in the seventh on Yoenis Cespedes’ RBI single. Michael Conforto homered in the 10th for the Mets’ other run.

In his return from the disabled list against the Yankees last week, Gsellman allowed two earned runs over 5 ¹/₃ innings and received the no-decision. The right-hander missed six weeks after straining his left hamstring running to first base in Miami.

Gsellman entered 5-5 with a 5.98 ERA, as perhaps the biggest underachiever among the Mets’ starting pitchers this season. He courted controversy before his return to the team by saying “I don’t really care” to reporters asking what he thought of general manager Sandy Alderson’s pronouncement he needed to start pitching better during his rehab assignment.

J.D. Martinez delivered an RBI single against Gsellman in the fourth to put the Mets in a 1-0 hole.