DStro asked the same question, as many of us have, and who it will be is really not any clearer.

From Mike Phillips:

Larry Beinfest, the Marlins' president of baseball operations, said he has a pitcher in mind for Sunday's start against the Phillies, and that pitcher is in the organization.

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The most likely candidate is Sean West, a 6-8 left-hander who was determined to make the team this spring. West, one of the Marlins' top prospects, pitched Tuesday for Double A Jacksonville and would be on track to make his next start Sunday.

Not sure I'm buying the Sean West theory, in that, I don't think the club will want to start his clock.

GameFish offered the following in the comments.

Joe Frisaro:

Hayden Penn could fit the bill since he's been a starter with the Orioles, and that may be where he is best suited. However, he hasn't been stretched out of late because he's been pitching long relief. Burke Badenhop, brought up to replace Miller's roster spot, could get the nod since he's made starts at Triple-A.

Perhaps the best option against the Phillies on Sunday, considering all the lefties on Philadelphia, is left-hander Graham Taylor, who is at Double-A Jacksonville. Taylor last pitched on Monday, so he would be well rested. Taylor wasn't in big league camp with the Marlins, but he pitched in the final Grapefruit League game.

Another Double-A option is lefty Aaron Thompson, a first-round pick in 2005, who last threw on April 17.

Juan C. Rodriguez

Right-hander Hayden Penn is not obsessed over whether the Marlins are considering him for Andrew Miller's spot in the rotation while he's on the disabled list. Penn's fine either way.

Starting Sunday against the Phillies, the Marlins will need someone to take at least two of Miller's turns while he recovers from a strained oblique. Penn has plenty of starting experience, but he's not stretched out.

Of course GameFish has her own ideas about who should start.

You can't spell Badenhop without H-O-P-E

I didn't quote box it because you need to read the whole thing.

The Marlins have some spots open on the 40-man roster so anyone in the organization is fair game, from that stand point.

If it were me, I think I would go with Meyer, assuming his arm is able to go at least five innings.  Since he is more or less a "veteran".

However, what ever happens it should be interesting come Saturday.