The Pittsburgh Pirates are a fast-emerging dark horse in the Jon Lester sweepstakes, joining the Los Angeles Dodgers in pursuit of the Boston ace as the Red Sox decide whether to trade him before Thursday's deadline, major league sources told Yahoo Sports.

Pittsburgh, two games back of the Milwaukee Brewers in the National League Central and a half-game behind three teams competing for the two wild-card spots, boasts one of the best farm systems in the game and could use it to fortify its run at a second straight postseason spot. And one name at the center of the talks, according to sources, is switch-hitting prospect Josh Bell.

The Red Sox continue to ask for an exceedingly high return considering Lester will be a free agent this offseason, sources said, targeting multiple high-end prospects in exchange for what amounts to a rental. The price is prohibitive, particularly with other aces, such as Tampa Bay’s David Price and Philadelphia’s Cole Hamels, nominally available, not to mention the strong perception that Boston must deal Lester now because it has no intentions of re-signing him in the offseason.

Bell, 21, is the sort of prospect around whom a deal could be made. He hit .335/.384/.502 at Class-A Bradenton before a recent promotion to Double-A. The Pirates signed him for $5 million after selecting him in the second round of the 2011 draft.

The Dodgers’ potential centerpieces are considered even more higher end: pitcher Julio Urias, outfielder Joc Pederson or shortstop Corey Seager. While the Dodgers are not insistent on adding a pitcher to a rotation that already includes Clayton Kershaw, Zack Greinke, Hyun-Jin Ryu, Josh Beckett and Dan Haren, the ability to use their vast financial resources and deep farm system to upgrade is indeed alluring, sources said.

Who that is, exactly, depends on how the market shifts before the 4 p.m. ET deadline Thursday. Tampa Bay, which has thrust itself back into the American League East race, could hold on to Price. Similarly, the Phillies, with Hamels under contract for another four seasons, may try to rebuild around him. Or one of the teams, sensing the glut of pitching and wanting to make a deal, could lower its asking price.

Boston could be the first team to budge. Lester is scheduled to start tomorrow night, and while the Red Sox at one point indicated they would like to know whether he’s staying or going before his outing, they expect teams will step up their offers over the next two days.

Other teams known to be interested in the frontline starters include the Brewers, Toronto Blue Jays, Baltimore Orioles, St. Louis Cardinals and Seattle Mariners.

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