The Brewers have been using a six man rotation since Matt Garza returned, but they were always going to switch back to the traditional five man rotation. It appears for now the winner is Mike Fiers. Andrew Gruman tweeted that Jimmy Nelson is being moved to the bullpen. Since he made a start recently it'll take a few more days of rest before he's ready to pitch again.
It's not really surprising that Nelson is the one moving to the pen. The two options were he or Mike Fiers. Over the last 30 days Nelson has pitched 33.2 inning with a 4.54 ERA, 3.48 FIP, and 4.01 xFIP. He's had a 16.8 K%, 6.0 BB%, .307 BAA, and a 1.51 WHIP. Oof.
Over that same time period Fiers has also pitched 33.2 inning with a 2.14 ERA, 1.99 FIP, 2.60 xFIP. He's had a 32.3 K%, 4.7 BB%, .174 BAA, and a 0.80. Yowzers! I knew he was really kicking ass, but to see those numbers really puts it in perspective. Fiers had been a true ace for a month (but let's not expect that to last forever).
So yeah, it probably wasn't a very hard decision for the Brewers to make. I'll be interested to see how Jimmy Nelson is used out of the pen. If he's limited to single inning outings he could really ramp up his already impressive fastball. The test is going to be pitching in back to back days. Also, it'll be interesting to see how quickly he can warm up to pitch. Marco Estrada takes a long time and if Nelson does too that could inhibit his overall effectiveness. We'll might learn some of these answers on Thursday when Nelson is first able to pitch again.
Statistics courtesy of Fangraphs