PHILADELPHIA -- Whatever one¡¯s definition of valuable might be, it¡¯s clear that Francisco Lindor embodies it. If any additional proof were required, the Mets' star shortstop supplied it Sunday through his absence. Attempting to play through a lower-back injury, Lindor lasted all of one inning before he again had to exit.

After that, New York¡¯s offense didn¡¯t do much of anything in a 2-1 walk-off loss to the Phillies at Citizens Bank Park.

¡°Obviously, he¡¯s a big part of the team,¡± designated hitter J.D. Martinez said.

The Mets received solid starting pitching in this one, as David Peterson carried a lead into the bottom of the eighth inning thanks to Tyrone Taylor¡¯s solo homer in the top half. But Peterson cracked for consecutive doubles to start the eighth, Edwin D¨ªaz allowed a walk-off single to J.T. Realmuto in the ninth and the Mets¡¯ offense -- minus Lindor -- couldn¡¯t do anything to counteract it.

But with the Braves losing to the Dodgers on Sunday night, the Mets departed Philadelphia still holding the final National League Wild Card spot, thanks to a tiebreaker edge over Atlanta.

With two weeks left in the regular season, Lindor¡¯s lingering absence is now the most significant question facing the Mets.

How long will Lindor stay sidelined?

The Mets won¡¯t know the answer to that until he goes for an MRI on his back, scheduled for Monday in New York. Lindor said he had been playing through back soreness for about two weeks prior to jamming it while lunging for and then overrunning second base on a hit in Friday¡¯s win over the Phillies. While he and manager Carlos Mendoza remain optimistic that Lindor can return before long, they can¡¯t predict with any accuracy if his absence will last another day, another few days or potentially even longer.

¡°It¡¯s early to sit here and say I¡¯m really worried,¡± Mendoza said. ¡°We¡¯ve just got to see how he feels tomorrow [and] the next day. It¡¯s too early to tell.¡±

Lindor did attempt to play on Sunday with a protective band around his lower back, even singling to lead off the game. But he began to feel pain creeping in as he ran the bases, and it later caused him to move tentatively as he fielded a ground ball in the bottom of the first. By the second inning, Lindor was feeling enough discomfort to ask out of the game.

¡°It¡¯s extremely difficult,¡± Lindor said. ¡°I¡¯m somebody who wants to post up and be available as many times as I can. It is difficult to come out of the game. It is difficult to not be there for my teammates, not be there for my coaches and play the game that I love. It doesn¡¯t feel good.¡±

Who might step up if Lindor¡¯s absence lingers?

A bona fide NL MVP candidate, Lindor is slashing .271/.342/.494 with 31 homers and 27 stolen bases. He has played in 148 of the Mets¡¯ 149 games, making him the beating heart of New York's lineup.

But plenty of other good hitters wear orange and blue, and if Lindor can¡¯t play, one of them will have to do more damage.

A prime example is Pete Alonso. By most any account, this has been a disappointing walk year for Alonso, who is batting .209 in September and whose only home run in his past 41 plate appearances came against a position player.

Overall, Alonso is slashing .241/.328/.465 with 32 homers -- solid numbers, but a half-step behind those of the game¡¯s elite sluggers. Even with a big final fortnight, Alonso may find his value on the open market diminished from what he once hoped it might be.

Still, Alonso has a chance to change the narrative surrounding his final year under team control. He¡¯ll face a pair of left-handed pitchers this week against the Nationals at Citi Field, which could help push him back into better form at the plate.

If Alonso can¡¯t take up Lindor¡¯s mantle, perhaps others can. Mark Vientos has cooled at the plate recently, striking out four times on Sunday, but Francisco Alvarez appears to be warming and is hitting the ball harder.

Then there¡¯s Martinez, who boasts some of the best batted-ball metrics in the Majors. Over a span of four at-bats on Saturday and Sunday, Martinez flied out three times on balls that traveled a combined 1,185 feet, all with exit velocities of 101 mph or greater (and one of which was quite literally headed over the fence before Cal Stevenson brought it back).

It¡¯s not a stretch, then, to think Martinez could make some noise over the next two weeks.

If Lindor¡¯s absence lingers, the Mets will badly need him to.

¡°It¡¯s just frustrating, you know?¡± Martinez said. ¡°It¡¯s almost like we lost the series, but it feels like we should have won the series. If those balls land, it¡¯s a completely different game.¡±