Byrd broke the news of his own trade to ESPNChicago.com's Bruce Levine, with the clubs both announcing the deal officially Saturday night.
The 35-year-old outfielder, who has also played for the Philadelphia Phillies, Washington Nationals and Texas Rangers, is an 11-year veteran and experienced center fielder with a .278 batting average. But he had just three hits in his first 43 at-bats with the Cubs this season (.070), with no extra-base hits and 10 strikeouts.
"He's been a good major league center fielder for a long time and he's off to a tough start,'' said Red Sox general manager Ben Cherington, who spoke with reporters late Saturday night. "Our hope is that a change of scenery and in a new environment we can get him going to help us and give (manager) Bobby (Valentine) another option in the outfield."
The Cubs will send cash covering nearly all of Byrd's pro-rated $6.5 million salary. To make room for Byrd on the roster, the Sox designated Nate Spears for assignment.
Byrd did not play in Chicago's game against the Cincinnati Reds on Saturday afternoon.
The trade took on added urgency when Red Sox outfielder Jason Repko went down with what Valentine described as a slight separation of his left shoulder, an injury incurred when Repko ran into the center-field wall Friday.
Cubs GM Jed Hoyer said Repko's injury did not directly lead to the trade.
"No, we've been talking to them since the end of spring training," Hoyer said in a conference call with Chicago media. "We've been trying to get relief pitching. In trading Marlon we felt we had outfield depth with some young guys that can play the position."
The Red Sox already have two elite outfielders on the disabled list, Jacoby Ellsbury and Carl Crawford. Repko was projected as a stopgap until Crawford's return, which is believed to be still a couple of weeks away.
"Obviously with Ellsbury out and Crawford still coming back, we felt there was a need to add to the outfield,'' Cherington said. "I think he knows he has to come in and perform to play, but it's a fresh start and I think he's excited to be here and we're excited to have him.''
Cherington acknowledged the difficulty of finding outside help at this time of year, which explains in part why the Sox are adding a player who has performed this season as poorly as Byrd has to date.
"We don't think his performance so far this year is reflective of who he is -- he's been a pretty consistent performer -- but teams are not anxious to give away good players this time of year, players who are helping them and are part of their future. It's a challenging time to make trades, but we felt this was a good fit for the team, made sense for the Cubs and Red Sox, and hope Marlon can help us.''
Last May 21, Byrd was struck near the left eye by a pitch by Red Sox reliever Alfredo Aceves and sustained multiple facial fractures, missing 39 games. At the time he was hurt, Byrd had posted a .308/.346/.419 line. After returning July 2, Byrd hit .255/.311/.380 for the rest of the season.
"He came back pretty quickly from it and performed pretty well when he first came back,'' Cherington said, when asked about the impact of the beaning. "And then he tailed off a little bit at the end of the season. I don't know. It's hard for me to answer that one. But physically, he's fine, he's passed all the tests. From a scouting standpoint, there's no obvious change in skills except his performance hasn't been there.
"We get to know him better when he gets here and we get him in the lineup and try to get him going.''
Bowden was once regarded as a promising prospect, first as a starter, then as a reliever, with brief stints in the majors in each of the last four seasons. But the Red Sox designated him for assignment on April 15, even though they have the highest earned run average in the major leagues.
"This underscores (Bowden's) slow start," Hoyer said. "Small samples shouldn't cloud your opinion on a player. We've known him since 2005. He's a local kid. I know he's excited to be coming to Chicago."
Crawford had left wrist surgery in the offseason and then strained his left elbow during spring training. On Opening Day at Fenway Park, on April 13, center fielder and offensive sparkplug Ellsbury suffered a subluxation of his right (non-throwing) shoulder and was placed on the disabled list.
Crawford has begun playing in extended spring games as a hitter but has yet to play in the field, though he could be out there soon, according to Valentine. Crawford hit a home run Saturday, Valentine said, and will be evaluated in Boston in the next few days before getting clearance to play in the outfield. Crawford's rehab has been overseen by former big league outfielder Brian McRae, who is a consultant to the Red Sox baseball operations department.
Ellsbury is expected to be lost for a minimum of six weeks, though he said earlier this week that surgery has been ruled out.
Repko's momentum caused him to crash into the wall after gloving the ball. Will he be placed on the DL?
"No," said Valentine haltingly. "Not today. We'll find out in a couple of days about 'slight' -- more 'slight' or less 'slight.' He has limited role duty (Saturday)."
In the meantime, Valentine has been forced to juggle his outfield. He had defensively challenged Cody Ross in center again Saturday, with Darnell McDonald in left and Ryan Sweeney in right. His outfield options off the bench were limited -- utilityman Spears and infielder Mike Aviles, who had worked out in right field in the offseason. First baseman Adrian Gonzalez also has a little experience in the outfield.
Information from Sahadev Sharma, who contributes regularly to ESPNChicago.com, was used in this report.