PHOENIX — The Yankees’ chances of winning the AL East are on life support.

Of greater concern is whether their bullpen meltdown Sunday was just a bad day or represents cracks emerging in a heavily worked unit.

Brent Headrick served up a grand slam to Nolan Arenado in the fifth inning, flipping the game on its head and all but wrapping up the Yankees’ spot in the AL wild-card series next week after an 8-4 loss to the Diamondbacks at Chase Field.

By dropping the series finale, a game they once led 4-1, the Yankees (89-66) fell to six games back of the Rays with seven to play.

They will welcome the Rays to Yankee Stadium on Tuesday for a four-game showdown, needing a sweep to stay alive in the division race. One win by the Rays in The Bronx will secure them the AL East title.

The Yankees would then turn their attention to securing the top AL wild card, ending Sunday leading the Red Sox by 5 ¹/₂ games, though Aaron Judge’s questionable status for that series because of a calf strain looms over everything.

In the meantime, Sunday’s loss — on the heels of Saturday’s walk-off win by the Diamondbacks (82-74) — was another regrettable one after third-inning home runs by Heliot Ramos, George Lombard Jr. and Spencer Jones had staked them to a 4-1 lead.

In the bottom of the fifth, Will Warren loaded the bases with one out, at which point Aaron Boone called on reliable lefty Headrick for his 79th appearance of the season — the second most in the majors.

Before Sunday, Headrick had inherited 50 runners this season and allowed only five of them to score.

But after striking out the first batter he faced, Headrick walked Corbin Carroll to force in a run, which brought Arenado to the plate.

Arenado crushed Headrick’s 2-0 fastball just to the left of the left field foul pole, but then straightened it out on the very next pitch (another fastball), clobbering a grand slam to put the Diamondbacks ahead 6-4.

The Diamondbacks later added a pair of insurance runs off John Schreiber in the seventh inning, the first earned runs Schreiber has allowed as a Yankee.