Byrd never lost consciousness but suffered a several-inch gash and swelling under his left eye. He was taken to a local medical facility, Massachusetts Eye and Ear, for evaluation. He remained in the hospital overnight to undergo a battery of tests to make sure he didn't suffer any broken bones or a concussion.
Byrd remained in the hospital on Sunday, and sources with knowledge of the injury said there was no timetable yet on his release.
"I just know talking to [Cubs trainer] Mark O'Neil that nothing was definitive," Cubs manager Mike Quade said after the game. "I know he took it pretty flush. It's something that is dangerous enough that they'll take a real good look at it."
After getting hit by the pitch from Red Sox starter Alfredo Aceves, Byrd lay on the field for several seconds before getting up and exiting with the Cubs trainers, who put towels over the left side of his face. The training staff got the bleeding under control in the locker room before Byrd left the ballpark.
"This is not a pitcher's arm that's a little sore," Quade said. "This is serious stuff."
Aceves also hit Cubs leadoff hitter Kosuke Fukudome in the side in the first inning.
Reed Johnson entered the game to replace Byrd, who has been hit by five pitches since May 11.
"It looked like he was in panic mode a little bit," Johnson said. "Whatever it is, he'll probably be back sooner than anyone expects. That's the kind of guy he is."
Cubs starter Carlos Zambrano hit Red Sox third baseman Kevin Youkilis in the back with a pitch in the fifth inning, prompting plate umpire Adrian Johnson to warn both benches. The Red Sox had a runner on first with one out when Youkilis was struck.
Bruce Levine covers the Cubs for ESPNChicago.com and ESPN 1000. Information from The Associated Press was used in this report.