"He's going to get an opportunity to play some first," Girardi said. "He's been a very good first baseman over the years, and he's going to get an opportunity to show us what he can do."
Overbay said that the deal essentially amounts to a three-day tryout, since he has an out clause in his contract on Friday.
"I know they've been watching me, but there's not a lot of opportunities out there, so I wanted to jump on this," Overbay said at George M. Steinbrenner Field. "This was an opportunity, and that's all it was. I'll go out there and see what I can do, and who knows? Stranger things have happened."
The move came on a busy afternoon for the Yankees, who also completed their trade with the Angels for outfielder Vernon Wells. Overbay shook hands with Wells, a former teammate with the Blue Jays, who told him, "I would show you around here, but I don't know where anything is."
Overbay was a non-roster invitee with Boston and batted .220 with seven RBIs in 19 spring games, but Mike Carp and Ryan Sweeney appeared to be ahead of Overbay in a battle to back up first baseman Mike Napoli.
A .270 career hitter over 12 big league seasons, Overbay played in 65 games last year with the D-backs and Braves, batting a combined .259 with two homers and 10 RBIs in 116 at-bats.
"Obviously, he knows how to play the position and he has played it at a pretty high level over the years," Girardi said.
The Yankees have been searching for help at first base following an injury to Mark Teixeira, who partially tore the tendon sheath in his right wrist and may not return until June.
Teixeira is said to have a 70 percent chance of avoiding season-ending surgery, and the Yankees have been auditioning non-roster invitee Juan Rivera at the position while he recovers. Dan Johnson is also in camp but has logged just two hits in 32 at-bats this spring.
After Tuesday, the Yankees have just four exhibition games remaining. They will wrap up the Florida portion of their spring with games on Wednesday and Thursday against the Orioles and Pirates, respectively, then will visit the Nationals in Washington, D.C., on Friday before traveling to West Point to play against the Army Black Knights on Saturday.
"We'll see how [Overbay] moves around, see how he swings the bat and try to make a quick judgment," Girardi said. "That's what we're going to have to do. We don't have a lot of time; we have five days, really, in a sense to make a decision. So we'll see."