Koronka was a candidate for the final roster spot if the team had decided to head north with 12 pitchers. Instead, they'll start the season with 11 pitchers, with right-handed starters Kerry Wood, Mark Prior and Wade Miller on the disabled list.
The arrival of Bynum, who can play both middle infield positions and all three outfield spots, made Theriot expendable.
Bynum made his Major League debut with the A's last season, batting .286 with one RBI in seven September games. Cubs general manager Jim Hendry said Bynum's versatility and speed caught the eye of the team's scouts last year.
"He's a very, very versatile guy," Hendry said. "He's possibly even better suited to our league, double-switch-wise. You know you can play him in five spots for sure, and play [him] off the bench. I thought it was just a good move for us to add another athlete."
Bynum stole 23 bases and hit .278 last year at Triple-A Sacramento, and as a left-handed hitter he will team with the right-handed Angel Pagan to give manager Dusty Baker more options off the bench.
"Pagan has really come on so well," Hendry said. "That gives Dusty a chance to maybe use Pagan in situations against lefties or off the bench against lefties, and then you've still got an athlete like Freddie now that you can do double-switches with and hit left-handed with and pinch-run with.
"We just thought it was a good gamble," Hendry added. "Hopefully it works out."
Baker said that while Bynum and other speedy offseason additions like outfielders Juan Pierre and Jacque Jones bring a new dimension to the team, it doesn't necessarily change his mindset as a manager.
"When you have [speed], you've got to use it," Baker said. "If you don't have it, you can't use it. But it's not a different approach. It's just some different options."
Patrick Donnelly is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.