Seibel joins right-handers Justin Speier and Chris Resop and left-hander Darren Oliver as new offseason pitching additions and likely will compete with Resop for the sixth spot in the bullpen.
The 27-year-old Seibel split last season at three levels in the Boston organization -- Class A Greenville, Double-A Portland and Triple-A Pawtucket -- and compiled a 6-3 record and a 1.25 ERA in 29 appearances, making 13 starts.
A graduate of nearby Cypress High School, Seibel missed the entire 2005 campaign recovering from Tommy John ligament-replacement surgery.
"Our scouts liked Seibel," general manager Bill Stoneman said. "He's rebounded from Tommy John surgery and put up some pretty good numbers in the Minors. We decided to go get him."
Seibel, who tops out at about 90 mph with good control and an above-average curveball, has a Minor League option left, which provides the Angels with some flexibility.
"He is going to take a run at making our staff as a left-hander out of the bullpen or on our [starting] staff this spring. This is a guy that has started and relieved both," Stoneman said. "But if he makes our club out of Spring Training, it is likely that he will make the club as a reliever."
Donnelly became a star with the Angels during the 2003 season, when he was named to the All-Star Game as a setup man and earned the victory in that year's win by the American League.
He went 2-2 with a 1.58 ERA that season in 63 appearances, but a freak accident when he was hit in the face by a fly ball during batting practice the following spring delayed his 2004 season, after which a series of neck and shoulder issues sapped his effectiveness.
Despite going 6-0 with a 3.92 in 62 relief appearances last year, Donnelly's role diminished over the past two seasons. Scot Shields emerged as the primary setup man to closer Francisco Rodriguez, while Hector Carrasco, signed last winter, also moved ahead on the depth chart as a middle reliever.
"In Anaheim there were not a whole lot of innings to go around. I think it was a win/win for everybody," Donnelly said. "There is a lot of appreciation for the organization. They were the first ones to give me a chance and have a career. I have a lot of friends there. I will miss some guys, but it's time to move on. I'm leaving a big rivalry with the A's and Angels, but I'm going to the biggest rivalry there is and I can't ask for more."
The 35-year-old Donnelly has a 23-8 mark with a 2.87 ERA in 277 Major League appearances and spent 10 years in the Minors before joining the Angels in 2002.
"When he wasn't pitching well and he wasn't pitching as much as he wanted, he wasn't happy about his inactivity," Stoneman said. "We've got a pretty solid bullpen right now. It looked like the opportunities for Brendan this year were going to be slim. If there weren't going to be opportunities for Brendan, then we could use him as a piece for getting something else."
Despite free-agent left-hander Barry Zito being on the market and Bartolo Colon still a month away from picking up a baseball as he rehabs from a partially torn rotator cuff, Stoneman said the Angels were not looking for more pitching.
"We're pretty well set for pitching, and am I happy with it? Absolutely," Stoneman said. "This should be a strong starting corps and relief corps."
Mike Scarr is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.