SEATTLE -- Three weeks: That’s all the time the Padres have left to earn a spot in the postseason.

And, suddenly, they have a serious problem at the back end of their rotation.

Were the Padres to reach the playoffs, it’s easy enough to predict the starting pitchers in a three-game Wild Card Series. Yu Darvish has been outstanding all season. Blake Snell has been excellent in the second half. Joe Musgrove, while shaky at times since the All-Star break, has mostly battled his way to quality outings. In some order, that trio would pitch.

But that’s a question, the Padres hope, for another day. Right now, their place in the playoffs is under serious threat. Despite a 6-1 loss to the Mariners on Wednesday afternoon at T-Mobile, San Diego’s lead over Milwaukee remained at two games when the Brewers fell to the Cardinals.

Mike Clevinger endured one of his roughest outings as a Padre, allowing six runs over five innings. He surrendered a pair of homers in the bottom of the first, and the Mariners had a 3-0 lead before they’d recorded an out. Then, in the fifth, Carlos Santana launched a back-breaking three-run homer, marking the first time Clevinger has allowed three home runs in a start.

“It was a challenge,” Clevinger said. “I was out of whack all day. They made me pay for it at the end.”

After the game, the Padres headed to Arizona, where they’re slated to begin a four-game series against the D-backs. For Thursday’s opener, they’ll hand the ball back to left-hander Sean Manaea, who was temporarily removed from the rotation because of his own struggles.

Lately, Clevinger has joined Manaea as a rotation question mark. In the past calendar month, his ERA has risen by from 3.47 to 4.47. In three September starts, Clevinger has allowed 15 earned runs across 13 1/3 innings.

“I’m just battling through some things right now,” Clevinger said. “I’m just trying to make it work with the best I’ve got. Some days, it’s there. Some days, it’s not.”

Said manager Bob Melvin: “There are times during the course of the game it looks really good. He’s just not sustaining it as long as he did a little bit earlier.”

There are 19 games and a pair of off-days remaining on the Padres’ schedule. Presuming the team wouldn’t let a starter pitch on three days’ rest during the regular season, San Diego will need at least seven more starts from the last two places in its rotation -- the places not occupied by Darvish, Snell and Musgrove.

Of course, Clevinger and Manaea were brought on board for precisely these types of starts. The Padres gave up resources to acquire both via trade. Both will become free agents after the season. If they aren’t going to make a significant impact now, then… when?

But the flip side of that question is this: Can the Padres -- with the Wild Card race as tight as it’s been -- truly afford to continue relying on two pitchers who have struggled?

Perhaps the obvious solution is that the Padres can continue to start Clevinger and Manaea, but their leashes must be short. These games are simply too important. Plus, San Diego’s bullpen is as healthy as it’s been all season. It hasn’t been taxed heavily either, mostly because the team was so reliant on its starters during the early part of the season.

“I think our bullpen is probably as strong as it’s been all year, and not overused to this point either,” Melvin said Tuesday. “... A lot of bullpens are probably worn out at this time of year. Ours is not, and that has a lot to do with the innings the starters have accumulated.”

Indeed, Melvin should have some flexibility to go to his bullpen early. But he chose not to on Wednesday. Clevinger unraveled quickly in the fifth, plunking consecutive Mariners batters. The Padres scrambled to get left-hander Adrian Morejon loose to face lefty Jesse Winker, who followed Santana in the order.

“And then, Santana is the guy that ends up taking him deep,” Melvin said. “At the end of the day, Santana was the big at-bat.”

So continued the strange up-and-down nature of the Padres’ season. They’re still waiting for it all to click -- still waiting for their first true stretch of authoritative, winning baseball. Considering the stakes, now would be as good a time as any.

“We’re going to need to make a little more of a sustained run here at some point in time in these last 20 games,” Melvin said. “We’ve said this. We keep saying it. We feel like we have the pieces to do it. We just haven’t done it yet. It’s time to just quit talking about it. We have to do it. Hopefully, in this next series, it shows up.”