PHOENIX — Bring back the varsity.

After a recent 7-3 home stand, which included games against division-leading clubs such as the Red Sox and White Sox, the Yankees headed west to face two struggling teams, the Diamondbacks and A’s.

It did not start well.

Chad Green was beaten up in his first big-league debut, the soft underbelly of the bullpen was exposed and Yankees bats reverted to their opening-month slumber in a 12-2 loss Monday night to the Diamondbacks in front of 32,718 at Chase Field.

“It was an awesome experience,” the 24-year-old Green said of stepping on a major-league mound for the first time. “I got some balls up and they did some damage with them.”

The Yankees (16-21) managed two runs and six hits against Arizona lefthander Robbie Ray, who came in 1-2 with a 4.84 ERA.

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“He threw strikes tonight, that was the biggest difference,” Joe Girardi said of Ray, who entered the game having walked 19 batters this season but walked just one last night.

Green, acquired along with Luis Cessa last December in the deal that sent lefty reliever Justin Wilson to the Tigers and getting the start with Luis Severino on the disabled list, allowed six runs — four earned — and eight hits in four-plus innings.

The righthander entered the fifth with the score tied at 2-2 but allowed a back-breaking three-run homer to Jake Lamb, who had three hits. The inning started with Michael Bourn attempting to bunt to get on. Green fielded and threw to first in time but Mark Teixeira dropped the ball for an error. Two batters later, Lamb launched a belt-high, full-count fastball to right to make it 5-2.

“I had him 0-2 and didn’t put pitches where I needed to,” said Green, who had not given up a homer in seven minor-league starts this season but allowed two Monday night. “I tried going in [to Lamb] and didn’t get it in.”

The Diamondbacks (18-23), who totaled 15 hits in snapping a five-game losing streak, added two more runs in the fifth, including Ray’s RBI single off Nick Goody, to essentially put it away at 7-2.

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They scored an unearned run in the seventh against Phil Coke and three earned in the eighth to make it 12-2. Goody and Conor Mullee preceded Coke to the mound and allowed one run each.

For all the talk about the Yankees’ sterling back end of the bullpen — Dellin Betances, Andrew Miller and Aroldis Chapman — the other relievers, except for Kirby Yates, haven’t consistently distinguished themselves.

“That was something I talked about in spring training,” Girardi said of the middle-inning relievers. “The three guys [we knew] were going to be really good. At times, it’s been a struggle . . . We need to find a combination that works.”

The Yankees’ bright spots on offense were limited to Teixeira’s three hits and Jacoby Ellsbury’s fifth-inning triple that led to the tying run.

Paul Goldschmidt put Arizona on the board in the first with a two-out homer, his eighth of the season, on a first-pitch, 94-mph fastball.

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The Diamondbacks tacked on in the second on an RBI double by Jean Segura, who had three hits.

The Yankees cut their deficit in half in the fourth when Teixeira led off with a double to left and went to third on Carlos Beltran’s sharp single to center. Chase Headley, coming off a 9-for-25 (.360) homestand, slapped an RBI single to right to make it 2-1.

In the fifth, Ellsbury drove a 2-and-0 fastball to the deepest part of the park, right-center, and as he dove into third, the relay throw from Ahmed, the shortstop, pinged off his helmet and out of play. That allowed Ellsbury, who collected his second triple of the season, to come in to make it 2-2.

“They swung the bats really well tonight,” Ellsbury said. “We were scoring runs during the homestand. One game isn’t going to waver us. We lost tonight, but we’ll come back tomorrow.”