The Yankees sent Joba Chamberlain to the mound for Sunday’s rubber match against the Mariners with the hope of stretching him back out for the playoffs.

The oft-criticized “Joba Rules” were lifted, and only the American League’s lowest-scoring offense stood between Chamberlain and his ninth win of the season. Yet the right-handed flamethrower, who turns 24 on Thursday, couldn’t make it past the third inning.

Consecutive doubles by Jose Lopez and Ken Griffey Jr. with two outs in the first and a single by Adrian Beltre plated two Mariners runs in the opening frame. And things only deteriorated from there, as Junior hit a three-run homer to highlight a five-run second inning for the home team.

All told, the Mariners posted seven runs (all earned) before Joba walked off the mound in the third. He allowed six hits — three of them for extra bases — walked three, and struck out just two. Clearly, Chamberlain’s blazing fastball and buckling slider weren’t fooling anybody in a 7-1 loss that blew his ERA up to 4.72 for the season.

That’s a significant concern for the Yankees, both because Joba is scheduled to pitch in a game with potential playoff impact against the Red Sox on Saturday, and because it remains unclear whether – and in what capacity – Joe Girardi can rely on him in the playoffs.

As the likely No. 1 seed in the American League, the Yankees will need to decide whether they want to play the ALDS that requires three starting pitchers or four. If Chamberlain can straighten things out, New York’s front four will be able to match up with that of any team – particularly the Tigers and Twins, who are both lacking in rotation depth. Conversely, should Joba’s struggles lead the Yankees to choose the three starter option, they might have to face Justin Verlander twice, while also trotting out the inconsistent A.J. Burnett.

For a team with no glaring weaknesses, the expensive starting rotation is starting to look like an Achilles’ heel for the Yankees as the playoffs draw near. All the offense in the world won’t save them if Girardi only has two reliable starters come October.