In exchange for the 24-year-old Pena, the Reds picked up right-hander Bronson Arroyo, a pitcher who went 14-10 with a 4.51 ERA last season. The Reds also received roughly $2 million from Boston to offset Arroyo's three-year, $11.25 million salary.
The trade boiled down to two teams that had excess pieces in one area and willing to trade one of those pieces to strengthen an area of weakness, Reds general manager Wayne Krivsky said.
Krivsky said the trade came about very quickly when the two sides realized that they had mutual interests that could be served.
"They've got about seven starting pitchers in their minds, and I would agree with that," he said of the Red Sox. "They're dealing from an area of surplus. They needed a right-handed bat. "
He said Pena, long considered a talent that was ready to blossom, would fit in nicely to hitter-friendly Fenway Park, where he'll probably play a platoon role.
Adding the 29-year-old Arroyo makes the Reds stronger, because a Major League team never has too much pitching, Krivsky said.
"He brings us a proven starting pitcher," Krivksy said. "The last two years, he's averaged roughly 190 innings. He's taken the ball every fifth day. He's never been hurt.
"I feel real good about the addition to our rotation."
So does manager Jerry Narron.
"For us, we get a solid Major League starter that's pitched in big games and knows how to compete," Narron said. "He's a guy who can throw a pitch over on any count. You've got to be able to do that.
"He's a guy who can give you 200 innings, which we definitely need. One thing that we've been missing is guys that go deep in the games. We think he's going to be able to do that for us."
Narron said he's made no decision on where Arroyo will fit into the rotation. He said Aaron Harang will remain at the front of it, and Arroyo should slide into the No. 2 or No. 3 spot.
"Whether he'll pitch that first time out before (Brandon) Claussen or after him, I don't know," Narron said. "But he'll be right there somewhere at the top of it."
In trading Pena, Narron will get a more settled situation in left field and at first base. He said Adam Dunn will move to left, and Scott Hatteberg will pick up at-bats at first base.
Dunn and Hatteberg improve the defense at both positions. Narron said he hated to see Pena go, but ...
"I like Wily Mo," Narron said. "I think everybody here does. But that's just part of the game. We're just trying to do everything we can to get better."