Milwaukee gets right-handers Carlos Villanueva and Glenn Woolard in the deal, which provides an immediate boost to San Francisco's banged-up pitching staff.
The 30-year-old Franklin went 10-13 with a 5.50 ERA in 34 starts for Milwaukee last year but wasn't going to make the Brewers' starting rotation this season. He becomes the Giants' sixth starting pitcher, including ace Jason Schmidt, who will miss his first start because of shoulder soreness.
The Giants don't need their No. 5 starter until the second week of the season.
San Francisco might choose to send down 22-year-old Jerome Williams, who's been struggling this spring. Williams, projected to be the No. 4 starter, was tagged for six earned runs and 10 hits in 3 1-3 innings of a 10-3 loss to the Chicago White Sox on Tuesday
and hasn't won in six exhibition outings.
The trade was in the works before Williams' latest rough start.
It was a busy day for the Giants.
Earlier Tuesday, they placed closer Robb Nen and reliever Scott Eyre on the 15-day disabled list and signed infielder Deivi Cruz,
right-hander Dave Veres and right-hander Mike Crudale to minor league contracts.
"This was less than 24 hours," general manager Brian Sabean said of how the trade developed. "It happened very fast. The optimum thing here was experience. We've got to have some people
starting the season with us who have been exposed to major league experience. They have a lot more experience than some of the alternatives we had in camp."
Estrella, 29, went 7-3 with a 4.46 ERA in 58 relief appearances
for the Brewers last season.
Both players are expected to join the team Wednesday in
Surprise, where the Giants play the Kansas City Royals in their
second-to-last Cactus League game before returning to the Bay Area
for their final three spring games. Estrella will probably pitch
against the Royals.
Franklin and Giants starter Brett Tomko allowed the most home
runs in the majors last season -- Franklin was No. 1 with 36 homers
allowed and Tomko was second with 35.
That's not a huge concern for Sabean, who believes Franklin will
do better in the pitcher-friendly confines of SBC Park.
"He's going to throw strikes and in our ballpark he can be more
aggressive," Sabean said. "The kid is still coming into his own.
... I feel better now that we have these two."