I believe it was a philosopher that once said, "give me your heart, make it real, or else forget about it". Zack Greinke would not give the Brewers his heart, so to speak, by signing off on a proposed $112 million contract extension, and GM Doug Melvin was left with no choice but to trade him.
And trade him he did, to The Los Angeles Angels of Anaheim (a name which literally translates to "The The Angels Angels of Anaheim") for SS Jean Segura and RHP's John Hellweg and Ariel Pena. The Brewers did well in this trade (better analysis than mine here and here), and hope still remains that Greinke could return in the offseason if the free agent market plays out that way.
Former first-round pick Mark Rogers will start in Greinke's place on Sunday, as fellow rotation candidate Tyler Thornburg is experiencing a sore arm. Rogers had a cup of coffee with the Brewers during the 2010 season and has overcome a slew of arm troubles to make it back to the majors. Rogers' seasonal number in AAA Nashville don't look all that impressive (6.99 K/9, 4.63 BB/9, 1.23 HR/9, 4.72 ERA), but he's pitched very well as of late, giving up just nine hits and two runs over his last 19 innings with 20 strikeouts.
As an aside, I have the vision in my head of Aaron Rodgers and Mark Rogers in a western-style showdown, with Aaron Rodgers drawling "this town ain't big enough for the two of us". Mark Rogers would almost certainly lose the duel, seeing as he would likely suffer a shoulder injury trying to draw the gun from it's holster.