Sources: BoSox ask about Justin Upton

The Red Sox have expressed interest in trading for Diamondbacks outfielder Justin Upton, multiple major-league sources told on Tuesday.

Discussions between the clubs are not far advanced, according to one person in the industry. The general managers of the respective clubs — Kevin Towers in Arizona, Theo Epstein in Boston — are close friends.

The Red Sox remain active on many fronts in their search for a hitter. They have interest in free agents Jayson Werth, Carl Crawford, Victor Martinez and Adrian Beltre.

The Diamondbacks are listening to offers on Upton, sources say, but Towers would need a “big” return to move him. Upton, 23, is signed to a reasonable contract that will pay him $49.5 million through the 2015 season.

Upton has a limited no-trade clause that is believed to include only five teams. He would appeal to a wide number of clubs, including the Atlanta Braves and Los Angeles Angels, both of whom have a number of intriguing young pitchers to include in an offer.

The fact that the Diamondbacks are willing to at least entertain moving Upton should give the Red Sox added leverage in discussions with agent Scott Boras, who represents Werth and Beltre.

Arizona would likely want three players for Upton — a backend reliever, a starter and a bat. But it’s not clear if there is a good match on players between the clubs.

Given their historical bullpen woes in 2010, the Diamondbacks would likely request ascendant reliever Daniel Bard; the Red Sox would probably rather move closer Jonathan Papelbon, who is more expensive than Bard and just one year from free agency.

Jacoby Ellsbury could fit Arizona’s need for a hitter. He would represent an upgrade over the incumbent Gerardo Parra. The Red Sox, though, might not have a major-league-ready starter that would appeal to the Diamondbacks. Lefty Felix Doubront might not be attractive enough, and righty Casey Kelly likely is off-limits.

Upton is coming off a down year — by his standards. He batted .273 with 17 home runs and 69 RBIs in 133 games. He played in only five games after Aug. 30 because of a left shoulder injury, batting just .143 during that span.

Arizona has more depth among its outfield prospects than at shortstop, where current starter Stephen Drew is seen as a trade candidate. Minor leaguers A.J. Pollock and Marc Krauss could reach the majors soon.