Hairston is a career .238 hitter with 16 home runs and 47 RBIs in 550 Major League at-bats.
"It's very nice to get an opportunity. To be going to San Diego, it's great," Hairston said. "I was excited about going to that organization. They're very successful. I know a couple guys already on the team. I've got a couple buddies on the team. It's going to be different, but change is good."
Hairston, once regarded as one of Arizona's top prospects, will be a backup outfielder for the Padres and a right-handed bat off the bench, which is something that the team currently doesn't have much of.
Hairston is expected to join the Padres in Houston on Saturday. At that time, the Padres will have to make a player move in order to put Hairston on the 25-man roster.
Hairston, a career .270 hitter against left-handed pitching, can also play second base in a pinch. That's the position at which the third-round pick in the 2001 First-Year Player Draft broke into the Major Leagues.
"He's handled the situation like a pro," Arizona general manager Josh Byrnes said of his conversation with Hairston. "He's a guy that's hit .320 in the Minors with power and has just never gone into a season with a big league starting job that was available. He got a lot of playing time this year. [Manager] Bob Melvin did a good job of keeping him in the flow particularly early."
Hairston is excited about the opportunity of joining the Padres. For what it's worth, he will get to see his now former Diamondbacks teammates on July 31, when the Padres open a three-game series at PETCO Park against Arizona.
"You really don't know what to expect this time of year, trade deadline's coming up, and when I saw the phone number on my caller ID and it was Josh, I knew something was up," he said. "That's the name of the game. It's part of the game, and the chances of a person sticking around the same organization are very slim. I understood that, but at the same time I'd really like to thank the Diamondbacks for giving me an opportunity first, and they were great to me, front office, coaching staff, everybody's been great."
The Padres currently have two reserve outfielders on their roster -- the switch-hitting Jose Cruz Jr. and Russell Branyan, who also plays third base.
But Cruz is hitting just .221 against left-handed pitchers and Branyan, a left-handed batter, has only 12 at-bats against left-handed pitchers this season.
Branyan committed an error and misplayed a fly ball in the Padres' 10-2 loss to Colorado on Wednesday.
Rosales, a 26-year-old right-hander, was placed on the disabled list on June 7 with a broken bone in his right hand. Prior to the injury, Rosales was 1-1 with a 3.28 ERA and 14 saves for Triple-A Portland.