General manager David Stearns said before Saturday's game with the Chicago Cubs that he does not know if surgery is needed. Nelson has a right rotator cuff strain and partial anterior labrum tear.
"Right now we know the mechanism of injury, which was sliding into the base," Stearns said. "We know generally what the injury is from the tests that we conducted this morning. In terms of treatment options, that is still to be determined."
Nelson left Friday night's 2-0 victory over the Cubs after the injury. The right-hander threw four-hit ball over five innings, while striking out seven and walking two en route to winning his third straight start. He is 12-6 with a 3.49 ERA, a career high for victories.
Manager Craig Counsell said he didn't yet know who would fill Nelson's spot in the rotation, which will come up five or six times before the end of the regular season.
"It's disappointing news," Counsell said. "I feel terrible for Jimmy. He worked so hard to kind of get himself into this spot. He took up his game to the next level this year."
Nelson singled off the left-field wall in the fifth inning on Friday and was hurt diving back to first after rounding the base. Initially, Nelson and Counsell did not think the injury was serious.
He said he felt discomfort pitching the bottom of the inning, and his velocity was down. Josh Hader came on to start the sixth for Milwaukee.
"It wasn't like extremely painful," Nelson said after the game. "It was just a weird feeling, just a weird kind of loose feeling, so it's just something where they didn't want to push it at that point."
Milwaukee entered Saturday four games behind first-place Chicago in the NL Central.