MIAMI -- The Miami Marlins entered this weekend with a potent lineup, a solid bullpen, a 1 1/2-game lead over the New York Mets in the battle for the National League wild-card playoff race and a very shaky rotation once you get past All-Star Jose Fernandez.

On Sunday, that rotation issue became much worse, and Miami's lead shrunk to a half-game over the Mets as a result of a 3-0 loss to New York.

Next up for the Marlins (53-45) is a visit from the Philadelphia Phillies, but Miami will have to take them on without starting pitcher Wei-Yin Chen.

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After Sunday's game, the Marlins announced that Chen, who they signed to an extravagant, five-year, $80 million contract in the offseason, was going on the disabled list due to an elbow injury.

There are rumors that Chen could be done for the season and may need Tommy John surgery. But, as of now, that's not what the Marlins are saying.

"(Chen) came in after his last start a little bit tender in the elbow," Marlins manager Don Mattingly said. "We've got to be cautious with him. We don't feel like it's anything crazy (such as surgery), but we also want to be careful."

Chen, who is 5-4 with a 4.99 ERA, has been a major disappointment for Miami. In fact, he hasn't been anywhere near the solid pitcher he had been during his four-year career with the Baltimore Orioles. Chen was 46-32 with a 3.72 ERA with the Orioles, which is how he got the big contract from Miami.

Unfortunately for the Marlins, Mattingly has no other good choice. He is about to replace one disappointing pitcher with another.

Jarred Cosart is expected to join the rotation in time to start Monday against the Phillies. Cosart, who started the season with the Marlins, was quickly demoted after he went 0-1 with a 7.98 ERA in three starts.

He has been mediocre at Triple-A New Orleans, posting a 3-4 record, a 4.09 ERA, a .158 WHIP and .279 batting average allowed.

This will be Cosart's first major league start since April 22, when the Marlins finally gave up on a pitcher who walked 15 batters in his first 14 2/3 innings this season.

Now, though, they need Cosart again.

"(Cosart) is the guy," Mattingly said. "He is throwing the ball the best down there (in New Orleans) right now."

That's not exactly a strong endorsement, but what else can Mattingly say?

The fact remains that the Marlins are one of several teams fighting -- at least for the moment -- for two wild-card spots. That list includes the Mets, St. Louis Cardinals and Los Angeles Dodgers. And don't forget the Pittsburgh Pirates -- they are just two games behind Miami.

Meanwhile, the rebuilding Phillies (45-55) are realistically out of the playoff race

Philadelphia, which lost to the Pirates 5-4 on Sunday, has dropped seven of 10 games since the All-Star Game. There is speculation that it won't be long before the organization calls up two top prospects -- 21-year-old shortstop J.P. Crawford and 22-year-old left fielder Nick Williams.

Phillies shortstop Freddy Galvis, who is batting just .232, could move to second base or to a utility role once Crawford arrives. Crawford, who is in his first year at Triple-A, is batting .317 with a .381 on-base percentage.

The Phillies on Monday will likely place infielder Andres Blanco on the disabled list after he was spiked by Gregory Polanco of the Pirates. Blanco's absence could open up a spot for Crawford.

Phillies third baseman Maikel Franco, who missed Saturday's game after getting hit by a pitch on his wrist, returned on Sunday.

The big need is for an upgrade at shortstop, which Crawford could address if he is ready.

The Phillies started Cody Asche in left field on Sunday, but he is hitless in his past 21 at-bats, including 10 strikeouts.

The solution could be Aaron Altherr, who was expected to be the Phillies starter in right field this season before he needed wrist surgery. The Phillies could also turn to reserves Tyler Goeddel or Jimmy Paredes.

Williams, who draws precious few walks, is another possibility, but he could probably benefit from remaining at Triple-A to hone his plate discipline.

On Monday night, Philadelphia will throw right-hander Jeremy Hellickson (7-7, 3.84 ERA) against Cosart, a matchup the Phillies no doubt love.

After all, Hellickson beat the Marlins 4-1 on Wednesday.