Hairston was batting .299 with 10 home runs and 29 RBIs in 56 games with the Padres this season, spending time in center and left field. His .891 on-base-plus-slugging percentage was the second-best mark on his team, behind only All-Star first baseman Adrian Gonzalez.
Hairston, who made 34 starts in center field and 14 in left this year, is expected to join the A's on Monday in Boston.
The 29-year-old was batting .327 in 43 games before being forced to the disabled list on June 2 with a strained left biceps. Since being reinstated on June 23, Hairston is batting .220 with two homers and six RBIs in six games.
Hairston, who made his Major League debut with the Diamondbacks in 2004, brings some power to the A's lineup, having hit a career-high 17 home runs last year and posted his fourth season with at least 10 homers in 2009.
The right-handed-hitting Hairston is a career .255 hitter with 51 home runs and 127 RBIs in 400 games with the Diamondbacks and Padres.
Webb was 7-1 with two saves and a 4.34 ERA in 31 games (two starts) with Triple-A Sacramento. The 23-year-old righty, selected by the A's in the fourth round of the 2004 First-Year Player Draft, has struck out 39 and walked 15 over 45 2/3 innings this season.
Italiano, 22, was 5-6 with a 5.63 ERA in 16 starts with Class A Stockton, striking out 75 and walking 40 in 76 2/3 innings. The right-hander was taken by the A's in the second round of the 2005 Draft.
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.