Cincinnati pitcher Bronson Arroyo, above, says he would be thrilled to play again for Boston.

AP / AL BEHRMAN

BOSTON — Red Sox general manager Theo Epstein admitted he made a mistake.

After the Red Sox traded utility outfielder Wily Mo Pena, along with cash considerations, to the Washington Nationals in exchange for a player to be named later on Friday afternoon, Epstein said he was wrong.

Not because he dealt Pena to Washington, but for acquiring him in the first place.

Pena was acquired by Boston from Cincinnati in exchange for pitcher and fan favorite Bronson Arroyo on March 20, 2006. Epstein was criticized for the deal back then. Friday, he admitted the move was a mistake.

“It didn’t turn out the way we wanted it to,” he said. “Certainly it wasn’t a good trade in retrospect. From a pure talent standpoint in a vacuum, that can still be debated because Wily Mo certainly has the talent and, in the right situation, he can hit a lot of home runs. There’s still a good chance he blossoms as a hitter. Here the fit never materialized.

“Certainly there have been times in the last 18 months where we could have used a very legitimate pitcher, which Bronson Arroyo is. We have to look back and say that [that trade] is probably one we would want back.”

While most, if not all, of Red Sox Nation would agree with Epstein, there was one particular person thankful for the public acknowledgment — Bronson Arroyo.

The former Red Sox and PawSox pitcher returned a phone call made by The Journal late Friday night in response to Epstein’s comments.

“I have a lot of respect for Theo,” said Arroyo. “He’s always been a straight shooter, and I appreciate that. To admit he made a mistake must have been tough to do. He has always treated me good.”

When asked if he would want to play again in Boston someday, Arroyo said: “Of course. I hope I have 10 years left in my career and at some point maybe I can get back there.”

Because of complexity of the rules at this time of the season, Epstein said he couldn’t reveal who the Sox had acquired from Washington, but said the bench will now be more functional this season.

“We’re pleased with the way things turned out [Friday],” said Epstein. “We ended up getting a player who we really like. We’re fairly happy with the resolution given the circumstances.”

jmcdonal@projo.com