01:00 AM EST on Tuesday, March 21, 2006

The question: Why would the Red Sox trade a reliable starting pitcher for a platoon outfielder?

The answer: Because in the long run, the Sox feel the platoon outfielder will prove to be a far better player than the reliable starting pitcher.

The next question: Are they right?

No one knows if Wily Mo Pena, acquired yesterday in a trade that sent Sox pitcher Bronson Arroyo to Cincinnati, will actually become the middle-of-the-order power source general manager Theo Epstein envisions. Everyone, from Dominican countryman David Ortiz ("He's a baby") to the analytical publication Baseball Prospectus ("[Pena] still makes the kinds of fundamental mistakes that he should have left behind in Double A"), agrees he's a work in progress. Sox fans saw that first-hand last June, when he went 1-for-10 with seven strikeouts in the Reds' three-game wipeout at Fenway Park.

But Baseball Prospectus also points out that Pena's development was retarded by the Reds, who only used him in 428 games, in both the majors and minors, from 2002-05. He never played more than 118 games in any of those seasons and hasn't had 500 at-bats in a single year since 2001, when he was at Class-A Dayton.

He won't this year, either, not if he's sharing right field with Trot Nixon. His raw numbers could improve dramatically if the Red Sox are successful in getting the majority of his plate appearances against left-handers -- a concept Cincinnati never mastered. But what of 2007, when Nixon probably will be gone and right field could be Pena's for the taking? And beyond?

The naysayers aren't hard to find. ESPN analyst Jeff Brantley, who admitted he's "never been a Wily Mo Pena fan," said yesterday a Pena at-bat is "a coin flip between a strikeout and a fly ball." He thinks the Reds definitely got the better of the trade. So do many of the people who participated in a projo.com survey soon after the deal was announced.

But the Web site baseball-reference.com uses an old Bill James statistical formula to compare players; the most similar players are called "comps." Pena's "comps" through age 23 -- which is how old he was last season -- are fascinating.

The biggest name of the bunch: Harmon Killebrew, the Hall of Fame first baseman. Others include 1960s sluggers Roger Maris, Willie Horton and Rocky Colavito, and 1970s stars Bobby Bonds, Dave Kingman and Willie Montanez.

The most-comparable player to Pena, according to baseball-reference.com, through age 23 was former Blue Jays and Yankees outfielder Jesse Barfield, who played 12 years in the bigs and hit 241 home runs. The most comparible player through 22 was Pete Incaviglia, another 12-year, 200-plus (206) homer man.

Almost every one of the 10 "comps" had a long and productive career in the major leagues. The only one who didn't was Billy Conigliaro, but even he once hit 18 homers as a part-time player (114 games) for the Red Sox.

These are the names the Sox had in mind when they made the trade. The names Epstein has in mind when he talks of Pena's upside.

It's not a given Pena will match, or even approach, the output of his "comps." He strikes out a lot -- an average of 154 times every 162 games so far in his major-league career -- and, as we saw with Mark Bellhorn, that particular weakness can sometimes destroy every other aspect of a player's game. He doesn't walk much, and lack of plate discipline is another weakness that will have to be addressed. As the old saying goes, if you swing at everything, eventually you get nothing to hit.

But he's only 24. He's already had some success in the major leagues. He'll have a strong mentor in Ortiz, his teammate on the Dominican Republic team in the World Baseball Classic.

"You know this game," Ortiz said. "It's a learning process."

He doesn't have to learn how to be Harmon Killebrew for the trade to be a success for the Red Sox. Being Willie Horton or Bobby Bonds would do just fine.

The Associated Press contributed to this report.

amartone@projo.com / (401) 277-7345