"He is a third baseman that [bench coach] Ty Van Burkleo and I spent last year with and we know his character and makeup," manager Don Wakamatsu said. "I think he will fit in real well. It gives us another option at third base along with [Chris] Woodward, gives us another left-handed bat and he's a good defender."
The Mariners put him to work immediately, starting him at third base in Saturday night's game against Rangers right-hander Kevin Millwood at Safeco Field.
"Getting sent down and getting that phone call and getting back up here, especially with Wak, it's awesome," Hannahan said.
The left-handed hitter recently was optioned to Triple-A Sacramento after struggling for the Athletics. He batted .193, hit one home run and drove in eight runs in 119 at-bats for the AL West club.
"We had the opportunity to acquire a premium defensive player who swings a left-handed bat," general manager Jack Zduriencik said. "Jack will become part of our mix at third base. He's a great make-up guy, and a well-above-average defensive third baseman."
He played in 143 games for Oakland last season, batting .218 with nine home runs and 47 RBIs.
Rangers manager Ron Washington knows Hannahan well.
"He's a solid player and certainly a professional," said Washington, a former coach with the A's. "I think Wak wants to get as many professionals over there as he can with [Adrian] Beltre being down and not being able to be as big of an influence as you would like to have in that clubhouse. He picked a good one."
Washington said Hannahan is "very solid at third base, can play first base, and gives you a good at-bat."
Beltre will be sidelined for at least five more weeks while he recovers from surgery to remove bone spurs from his left shoulder.
On Friday, the Mariners traded shortstop Yuniesky Betancourt to the Royals for two Minor League pitchers.
Souza, a right-hander, was the Mariners' ninth-round selection in the 2006 First-Year Player Draft. He split the 2008 season between Class A Wisconsin (3-3, 3.69 ERA), High Desert (2-1, 4.31) and Triple-A Tacoma (0-0, 2.08).
He spent this season with Double-A West Tennessee in the Southern League, compiling a record of 6-6 with a 3.35 ERA in 20 games, 14 starts.