One of the unfinished pieces of business on Sandy Alderson’s desk after the non-waiver trade deadline was the need to acquire a reliever or two to help Addison Reed and Jeurys Familia nail down games.

A seventh-inning guy, if you will.

For now, that guy is Hansel Robles. Called in to protect a one-run lead on Tuesday night, Robles gave up three runs to the Diamondbacks and the Mets went on to a 5-3 loss before 31,884 at Citi Field.

The Mets have still not won consecutive games since July 6-7.

Citi Field was buzzing after the red-hot Neil Walker hit a go-ahead two-run home run to right-center off Zack Greinke in the sixth inning.

The Mets were trailing 2-1 when Walker – who hit the eventual game-winning two-run homer in the ninth inning on Sunday in Detroit – followed Curtis Granderson’s leadoff double with his 20th home run of the season.

Walker (3-for-4), a free agent after the season who Alderson said before the game he would like to re-sign, is batting .491 (26-for-53) with four HRs and 11 RBIs in his last 13 games.

But Robles (5-4) allowed a two-out, two-run triple to Michael Bourn in the seventh to give the Diamondbacks a 4-3 lead. Manager Terry Collins left Robles in to face the lefthanded-hitting Bourn instead of calling in Jerry Blevins.

Collins cited the numbers: Bourn went into the game batting .358 vs. lefties and Robles had held them to a .170 average.

“It was a pretty good matchup,” Collins said. “He just didn’t make the pitch.”

Paul Goldschmidt followed with an RBI single off Robles to give the last-place team in the NL West some insurance.

The Mets are in another one of those “crucial stretches” managers talk about. Starting on Tuesday, they face two of the worst teams in baseball exclusively for nine consecutive days. It’s Arizona, then San Diego, then at Arizona.

Mets starter Steven Matz overcame a 28-pitch first inning to throw six innings and a career-high 120 pitches. He allowed two runs and five hits with one walk and nine strikeouts (tying a career high) and was in line for the win when Walker went deep.

With the Mets leading 1-0 on James Loney’s first-inning RBI groundout, Matz allowed a leadoff home run to Brandon Drury in the second. Goldschmidt gave the Diamondbacks a 2-1 lead with an opposite-field shot in the sixth.

In his previous 12 starts, Matz had gone 1-7 with a 4.54 ERA.

Oh, those batting averages with runners in scoring position. Alderson lamented the Mets’ major-league worst .204 average in such situations before the game. He also stressed that it could be an area of improvement down the stretch.

But you have to put runners in scoring position before you can have a chance to drive them in. The Mets were 2-for-17 combined in losses on Friday and Saturday in Detroit. Then they put no runners in scoring position on Sunday, but won behind home runs from Walker and Michael Conforto.

The formula seemed similar on Tuesday. Against Greinke, the Mets only had two at-bats with RISP in the first five innings: Loney’s RBI groundout and the next batter, Wilmer Flores, who ended the first inning with a runner on third by hitting a looper to short.

Greinke was making his first start since June 28. He had been on the disabled list with a left oblique strain.

Walker’s home run made the Mets 1-for-3 with RISP. They finished 1-for-4.

After Walker’s homer, Arizona pitchers retired 13 of the final 15 batters. The only hit was a two-out, eighth-inning single by – who else? – Walker. But Enrique Burgos struck out Jay Bruce to end the inning.

In the ninth, Alejandro De Aza appeared to beat out a one-out chopper to third, but the safe call was overturned on a replay review. Rookie closer Jake Barrett walked Conforto, but retired pinch hitter Ty Kelly on a grounder to short for his fourth save.