To acquire Byrd, the Red Sox sent right-hander Michael Bowden -- who was designated for assignment last week -- and a player to be named to Chicago.
"You know, he's been a good Major League center fielder for a long time and he's off to a tough start, but our hope is that a change of scenery, maybe a new environment, we can get him going and help us and give [manager] Bobby [Valentine] another option out in the outfield with [Jacoby] Ellsbury out and [Carl] Crawford still coming back," said Red Sox general manager Ben Cherington. "We felt there was a need to add to the outfield, and hopefully, he can help us."
The Cubs, with Theo Epstein as president of baseball operations and Jason McLeod as assistant GM, are highly familiar with Bowden. They selected him in the 2005 First-Year Player Draft with the Red Sox.
Byrd, who didn't play for the Cubs on Saturday, is off to a tough start at the plate, having hit .070 in 43 at-bats. All three of his hits have been singles.
"Well, we don't think his performance so far this year is a reflection of who he is," Cherington said. "He's been a pretty consistent performer. Teams are not anxious to give away good players this time of year, players that are helping them and players that are a part of their future. It's a challenging time to make trades, but we felt like this was a good fit for the team, made sense for the Cubs and the Red Sox. We hope Marlon can help us."
The Cubs will cover most of Byrd's $6.5 million salary for 2012. The outfielder is in the final year of his three-year, $15 million contract.
The 34-year-old Byrd is a proven Major Leaguer who has been productive in the past, and he's a plus defender in center field.
"The Red Sox came to us about Marlon, because they wanted help in center field," said Cubs GM Jed Hoyer. "We wish him well there. We think he can add a lot to their team. He'll go there incredibly motivated, and I think he can help them."
The Red Sox are without Crawford and Ellsbury, the latter of whom could be sidelined until at least June because of a subluxation of his right shoulder. Crawford is playing in games at extended spring camp, as he recovers from left wrist surgery and recent left shoulder inflammation.
The Red Sox have been playing Cody Ross, Ryan Sweeney and Darnell McDonald in the outfield. Jason Repko, who was called up from Triple-A Pawtucket last week, has also been in the mix, but he suffered a shoulder injury in Friday's 6-2 loss to the Yankees and is day to day.
Byrd had an unfortunate experience at Fenway Park last season, when Red Sox right-hander Alfredo Aceves hit him just above the left eye with a pitch. Byrd was out of action from May 22 to July 1 and wore a helmet with an extra protective flap on the left side upon his return.
From the research Cherington has done, he doesn't think it has contributed to Byrd's slow start.
"He came back pretty quickly from it and performed pretty well actually when he first came back," said Cherington. "He tailed off a little bit at the end of the season and then has had a rough start, so I don't know, it's hard for me to answer that one. Physically, he's fine. He's passed all the tests. From a scouting standpoint, we've watched him and there's no obvious change in skills. Just performance hasn't been there. We'll get to know him better when he gets here, and we'll get him in the lineup and try to get him going."
In his career, Byrd is a .278 hitter with 81 homers and 438 RBIs. Byrd's best season was with the Texas Rangers in 2009, when he hit .283 with 20 homers and 89 RBIs.
Byrd will join the Red Sox for Sunday night's game against the Yankees, and will likely start with lefty CC Sabathia serving as the opposing pitcher.