Rumors surrounding the availability of Arizona Diamondbacks right fielder Justin Upton continue to gain steam Sunday.

According to Jon Morosi of FOXSports.com, the NL Central Pittsburgh Pirates reportedly have interest in the D-backs slugger.

Major-league sources told FOXSports.com the Pirates have had preliminary trade discussions regarding two power-hitting outfielders, San Diego's Carlos Quentin and Arizona's Justin Upton.

Upton, who finished fourth in the MVP voting in 2011, has struggled to build upon the success he enjoyed last season and has failed to meet club officials' lofty expectations so far this season. However, Upton remains highly regarded around the majors despite falling out of favor with the Diamondbacks' brass of late.

Upton's relationship with the Arizona franchise has soured recently amid criticism from owner Ken Kendrick, but he's highly regarded within the industry. A change of scenery may be the best thing for Upton. If he lands in Pittsburgh, he would combine with McCutchen to give the Pirates their best collection of star talent since the departure of Barry Bonds.

If the D-backs decide to seriously entertain offers for Upton, they will have no shortage of suitors. In fact, the 24-year-old outfielder's services are so highly coveted, teams are already believed to have expressed a strong level of interest in obtaining the talented slugger.

Baseball Insider Jon Heyman of CBSSports.com reports the D-backs will only listen to offers that will net a huge return.

The Diamondbacks, who are on the cusp of the race in the NL West, don't have to trade Upton and are said only interested in making a "perfect'' deal. But it was recently reported by FOXSports.com the team might entertain offers.

Upton's no-trade list reportedly has changed from the last time the D- backs considered dealing him.

Sources say Upton's limited no-trade list isn't the same as the last time the Diamondbacks entertained trade requests a year and a half ago. It was reported at the time that his four-team list of teams he could veto a trade to consisted of the Tigers, Indians, A's and Royals. Detroit is one team that would have the interest and wherewithal to do a deal.

The defending NL West Champions are four games out of first in the division entering the All-Star break. It remains a long shot the Diamondbacks will deal Upton while still involved in a playoff race.

Upton is hitting .273 with seven home runs and 37 RBI and is in the midst of a six-year, $51.25-million contract that runs through 2015.