NEW YORK -- If there is one blot of adversity the Mets can ill afford to face, it is an injury to Jacob deGrom. In the views of many, deGrom has risen to the title of undisputed best pitcher in the world. Even for a Mets team that invested so much to improve all aspects of its roster this winter, deGrom remains the most indispensable piece.

That is why the Mets observed such caution in the sixth inning Sunday at Citi Field, when deGrom departed due to what team officials called a bout of ¡°right side tightness.¡± The episode occurred six days after deGrom skipped a start due to inflammation in his right lat, a major muscle on the side of the body. And it cast storm clouds over what was otherwise an uplifting fifth straight win and series sweep for the Mets, who relied on another strong bullpen performance to topple the D-backs, 4-2, and retake first place in the NL East.

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After the game, deGrom went for an MRI, which the Mets hope will reveal further information.

¡°We¡¯re hoping for the best,¡± said outfielder Michael Conforto, whose leaping catch in the second inning provided an early highlight of the win. ¡°Jake¡¯s very careful. He¡¯s not going to pitch on anything that doesn¡¯t feel right. He¡¯s a really tough guy as well. He¡¯s strong. He¡¯s built well. We don¡¯t know anything at this point, so it doesn¡¯t make any sense to speculate what¡¯s going on. But obviously, we don¡¯t like to see him leave the game early.¡±

deGrom was unavailable for comment because he was traveling to Manhattan for his MRI.

He pitched the first five innings without major issue, looking imperfect at times but nonetheless managing to retire the first dozen batters he faced. As the game progressed, so did deGrom, who struck out four over the third and fourth innings upon recovering from an early inability -- relatively speaking -- to spot his fastball where he wanted. deGrom even contributed his league-high seventh hit as a pitcher, bunting for a single that furthered the Mets¡¯ two-run rally in the third.

But deGrom¡¯s run of excellence ended in the fifth, when he walked three batters in an inning for the first time in three years. He escaped that jam having allowed just one run to raise his league-leading ERA to 0.68, yet as deGrom began throwing his warmup pitches in the top of the sixth, he called for head trainer Brian Chicklo to meet him on the mound. The two briefly conversed before deGrom departed.

¡°We were debating, and it wasn¡¯t worth it to take the risk,¡± Rojas said. ¡°That¡¯s when we went to remove him, to take a closer look at what¡¯s going on there.¡±

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According to Rojas, that marked the first time deGrom had complained of any tightness during the game. The issue was located more toward his lower back than his lat, which eased some initial concerns that the Mets had brought him back too soon. (deGrom had managed to play catch multiple times and throw a bullpen session after his missed start without issue.) Still, the Mets are plenty aware of how intricately those muscles are connected.

¡°I want to leave that to the medical staff and our team doctor, so they take a closer look and see what it is,¡± Rojas said.

Miguel Castro entered in relief of deGrom, taking all the time he needed to warm up because of the injury. He and two other relievers combined to squelch any thought of an Arizona comeback, with Edwin Díaz locking down a five-out save.

Those were all good things for the Mets, who managed to sweep the D-backs despite a rough start from David Peterson on Friday night, a dustup between two of their best players -- ¡°rat or raccoon?¡± suddenly seems ages in the past -- later that evening, and an injury to one of the game¡¯s best players in Sunday¡¯s finale.

The latter issue is by far the most pressing of the three, even if it winds up proving minor. deGrom has endured several back, lat, and neck muscle issues over the past few years, but none serious enough to cost him significant time. And the Mets are hopeful this will prove similar. Rojas went out of his way to stress that deGrom felt no pain on Sunday, only tightness, as he warmed in the sixth.

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Rojas and other team officials will soon learn more; if deGrom is unable to make his next start, Jordan Yamamoto would be the most obvious candidate to pitch in his place. Longer term, Carlos Carrasco and Noah Syndergaard are both due to rejoin the rotation in June.

The Mets hope deGrom will have long since returned by that point. And if he hasn¡¯t? They¡¯ll figure something out.

¡°It¡¯s not ideal to see our ace go down in the middle of the game, with our bullpen stretched the way that they have been over the past couple of games,¡± Conforto said. ¡°It¡¯s definitely not ideal. But like you¡¯ve seen, we¡¯ll find a way to win one way or another.¡±