The Brewers made the move Sunday, two days after Chacin fell to 3-7 with a 5.74 ERA after giving up seven runs in 2 2/3 innings in a loss to Pittsburgh.
Milwaukee recalled right-handed relievers Jacob Barnes and Taylor Williams from Triple-A San Antonio to give the team some help in the bullpen for Sunday's series finale against the Pirates, though neither player was used in the 4-2 Brewers win. The Brewers optioned catcher Jacob Nottingham to Triple-A.
The team said it expects to recall Nelson on Wednesday and start him against the Miami Marlins. The appearance will be Nelson's first in the majors since Sept. 8, 2017, when he partially tore his labrum and strained the rotator cuff in his right (throwing) shoulder diving head-first into first base to beat out a single against the Chicago Cubs.
The injury put an end to Nelson's breakout season. He went 12-6 with a 3.49 ERA for Milwaukee in 2017. He underwent shoulder surgery shortly after the injury and missed all of 2018.
Nelson went 3-0 with a 3.75 ERA in three starts and one relief appearance for San Antonio this spring. He will make his return to the majors on his 30th birthday.