Rosales is likely to be designated for assignment prior to Wednesday's matinee against the Blue Jays upon Callaspo's arrival.
"We needed some infield help, particularly someone who could help swing from the right side," A's general manager Billy Beane said. "We're pretty thin infield-wise, when you start thinking about the fact we've played Josh [Donaldson] pretty much every inning of almost every game. He can give Josh a breather. He can hit against left-handed pitching, play some second base."
Beane didn't anticipate any other moves before Wednesday's 1 p.m. PT Trade Deadline, saying he was "not currently" engaged in other trade talks.
"Sometimes things happen quickly," he said, "but I would say probably not. You never know. You get calls sometimes early in the morning on the last day, but at this point I don't see anything right now."
Callaspo, 30 and in his eighth Major League season, has strictly played third base in each of the last three seasons. He hasn't played at second since 2010 and hasn't been a regular there since 2008, but he offers the A's a better bat against left-handers than Rosales, who was hitting just .197 entering Tuesday.
For his career, Callaspo has favored the right side of the plate, posting a .763 OPS next to a .696 clip as a left-handed hitter.
The second-base platoon likely stays intact, then, with Sogard and Callaspo sharing duties at the position, leaving Jed Lowrie to remain at shortstop on an everyday basis going forward, with Sogard acting as his backup.
Beane, who began fielding calls from the Angels on Monday morning, didn't shy away from questions regarding Callaspo's absence from second base, stating up front, "I am concerned."
"But he has played it a lot in the past, and there wasn't a huge availability of that type of player," he continued. "He's going to have to get over there. We were in a situation where we needed another infielder."
Callaspo hit .252 with five home runs and 36 RBIs in 86 games for the Angels, having also compiled 34 walks next to just 22 strikeouts spanning 335 plate appearances.
He is owed roughly $6 million, including $4.875 million in 2014, and the A's are expected to pay all of it.
"I think he's a great player," said Chris Young, a teammate of Callaspo's in 2007 in Arizona. "He's really good defensively, and he's a guy that's going to put the ball in play. Great bat control, one of the better bat control guys in the league. I think he has a great approach at the plate and is able to make a difference up there and keep things moving."
"He's a quality player," said Donaldson. "He's one of those guys that can be a table-setter and get on base a lot. He's solid in all parts of the game."
Green, 25, made his Major League debut for Oakland on July 8, ultimately going hitless in 15 at-bats spanning five games while committing three errors. Defense has always been a concern for Green, whose movement between positions in the Minors slowed his journey to the big leagues.
However, the A's always thought highly of his bat, which registered a .325 average with 27 doubles and 11 home runs in 87 games for Triple-A Sacramento this year. He was a late scratch from the River Cats' game on Tuesday night, around the same time Callaspo was pulled from the Angels' game in Texas.
Beane admitted giving up a first-rounder wasn't easy, but it wasn't "so much a matter of things not working out with him."
"It's just a matter of addressing a need for a first-place team and recognizing and trying to grasp the present," he explained. "We're in a pretty unique position. I think we need to do everything we can to help this club out. The guys here, Bob [Melvin], the staff and the people that follow this team deserve that. We did think highly of Grant, and the price was steep, but it was a sellers' market for this position, and not many guys out there, but we needed to get somebody."
It's a sentiment appreciated by his employees.
"In a clubhouse, no matter what happens, you believe you have what it takes," Young said. "We've been able to play pretty well this season, but any time a move is made in an attempt to make the team better, you have to respect it."
Said Sogard: "Any way we can improve, absolutely. We're looking forward to this last stretch, and hopefully we can keep this going. [Callaspo] is a great player. He's been a hassle for us, so it's great to have him on our side. I think he'll be a great fit in our clubhouse."