Having thrown a couple innings yesterday, and with the Cubs perhaps needing to cover a lot of bullpen innings today, lefty Randy Rosario is headed back to AAA Iowa for now.
In his place, the Cubs have called up righty Alec Mills, who has periodically flashed big league ability out of the bullpen and in spot starts. As far as the 8th-man rotational bullpen arms go, Mills is perfectly fine to bring up and down as necessary. He could see a couple innings of work today.
As for Rosario, the likely other reason he was chosen to go down today is because Mike Montgomery is due back soon from his lat injury, and Rosario was likely going to be the guy he replaced anyway. So, going this route, the Cubs get an extra arm for the next few days at least (or until whenever it is that Montgomery returns).