Branyan had a previously undisclosed oblique injury and had seen his playing time diminish drastically of late in manager Ned Yost's soft but crowded platoon at third base with Bill Hall that also featured left-handed hitting Craig Counsell quite a bit during the past two weeks.
Nix is up to Milwaukee for the third straight season for what he hopes to be more than a drink of water this time.
The former Texas Ranger had been called up in 2006 and '07, but injuries and strikeouts had curtailed his big-league advancement.
He struck out 11 times in 35 official at-bats in 2006, when he hit .229, and K'd four times while going 0 for 12 last year before heading back to Triple AAA Nashville.
That's where Nix had been all of this season, overcoming a slow start to finish play Tuesday with a .284 average, team-leading 23 homers and 60 RBIs. He had hit safely in 17 of his last 20 games, posting a .364 mark with 15 extra-base hits.
Nix is a true center fielder but has played in left and right and gives Yost more flexibility and another left-handed stick, especially while Ryan Braun remains sidelined.
It will be interesting to see whether Nix can provide any consistency at this level and help the Brewers during their march toward a potential postseason berth.