The Padres and Red Sox have reached an agreement on players in a trade that would send All-Star first baseman Adrian Gonzalez to Boston for four minor league prospects, sources confirm.

Boston has until 2 p.m. ET Sunday to sign Gonzalez to a contract extension.

Sources also confirmed that Gonzalez is in Boston working on an extension, and that the Red Sox are the team he wanted to go to all along, so the likelihood the trade works out seems strong. Gonzalez is seeking a contract similiar to the one Mark Teixeira received with the Yankees after the 2008 season, $180 million over eight years. Phillies first baseman Ryan Howard, another comparable player, recently signed a five-year, $125 million contract. Gonzalez has one year to go on his contract with San Diego for about $6 million.

It isn't known whether Boston would go through with a trade even if it can't come to a quick extension with Gonzalez. The Red Sox might be willing to do a deal without an extension as long as the parameters are reasonable to them. It's also possible the Sox could wait until spring training before committing to a monster deal considering Gonzalez had surgery on his non-throwing shoulder in October.

Gonzalez's agent, John Boggs, said he is "well ahead of schedule.'' Boggs also said, "I think he's going to be ready for Opening Day. I don't think there's any doubt about that.''

Gonzalez underwent a complete physical Saturday morning in Boston, and while the shoulder is still healing, it "should be fine," according to Comcast SportsNet New England.

If the trade falls through for some reason, the Padres would keep Gonzalez into the season rather than continue to field offers.

The deal lessens the already slim chances that star third baseman Adrian Beltre is re-signed by the Red Sox. Boston is likely to move Kevin Youkilis to third base, where he has been working out this winter, although they could also move Youkilis to left field. The Oakland A's have made a big offer to Beltre, and the Angels and others also are very interested.

The Red Sox have long sought Gonzalez as the answer to their offensive puzzle and had the prospects to make it happen. One player in the deal, according to sources, is Casey Kelly, a 21-year-old pitcher who went 3-5 with a 5.31 ERA for Double-A Portland in 2010. First baseman Anthony Rizzo and outfielder Reymond Fuentes, who is Carlos Beltran's cousin, are the other two players, sources confirmed, while a fourth player will be named later. Rizzo, 21, batted .263 with 20 home runs at Double-A Portland last year and Fuentes, 19, batted .278 with 42 stolen bases at Single-A Greenville.

While the Padres contended all last year and ultimately finished a game behind the eventual World Series champion San Francisco Giants, they seek to keep their low payroll down and have been shooting for top prospects in any Gonzalez deal.

The Red Sox and Padres have been discussing Gonzalez for more than a year. The Padres' front office is very familiar with Boston's prospects, as GM Jed Hoyer, assistant GM Jason McLeod and new top executive Josh Byrnes all once worked for Boston.

Gonzalez, the No. 1 overall pick in the 2000 draft, has spent the past five years in San Diego, his hometown, and in the past four seasons has had at least 30 home runs and 99 RBIs.

ESPN and the Boston Globe first report the sides appeared to have agreed on the players in the trade.

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