Guerrero also threw out the first pitch on opening day at Angel Stadium, and new Angels hitting coach Don Baylor had to be helped off the field after injuring his right leg while catching it.
Guerrero signed the contract on the field before the game, getting a loud ovation from the Anaheim crowd when he put pen to paper.
The slugging outfielder then threw the first pitch to Baylor, the only other league MVP in Angels history. But Baylor couldn't get up after making the catch, unable to put weight on his leg.
Two Angels staff members had to help him off the field. The former Rockies and Cubs manager was taken to a hospital for further tests while the Angels faced the Seattle Mariners.
The 39-year-old Guerrero hasn't played in the majors since 2011. He spent six of his 16 major league seasons with the Angels from 2004-09, winning the 2004 AL MVP award and earning four of his nine All-Star selections.
Guerrero began his career with eight seasons for the Montreal Expos and finished with single years in Texas and Baltimore. He played briefly in the minors for Toronto in 2012, and was out of baseball last year.
Guerrero leads all Dominican major leaguers with 2,590 hits, also finishing with 449 homers and 1,496 RBIs. He had 12 seasons with at least 25 homers.
He never batted lower than .290 in his final 15 seasons despite a well-known penchant for swinging at balls outside the strike zone. Guerrero hit above .300 for 12 straight years.