Nelson was already en route to Miami. He will work out with the club but cannot be in uniform or sit on the bench for Saturday's game. The move will not become official until a corresponding decision is made to make room on the roster.
The 24-year-old Nelson will make his 2014 debut after going 5-1 with a 1.71 ERA in nine starts for Triple-A Nashville. He posted a 0.90 ERA in four games (one start) over 10 innings as a September callup last season.
Gallardo, who threw off flat ground Friday afternoon after missing a bullpen on Thursday, sustained a sprained left ankle on Tuesday in his previous start.
"I know myself pretty well that it's not bad," Gallardo said before the decision was made. "It's nothing that's going to alter some of the things I do on the mound or stuff like that. I'm satisfied with the flat ground I threw today. There's a lot more things that go into it than just standing on the mound, but I think jogging, running. It's a little stiff, but nothing that I can't get through."
Roenicke had acknowledged that Gallardo's session went well, but he felt uneasy about the possibility of a Marlins player bunting the ball early in the game. Gallardo did not field on Friday and was not shagging in the outfield during batting practice.
"There's still reasons why we're a little bit concerned with having something controlled like a bullpen and being in a game where they can bunt on him, he's got to cover first, he's going to have ground balls hit right back at him, he's got to run the bases," Roenicke said.
"We're in trouble getting through that game and the next day and the day after. That's probably the biggest concern. I think he's at a point where I'm not so worried about his arm as much. I think he'll land well with the ankle and he won't alter how he's pitching according to how his bullpen went today, but it's the other stuff not being able to complete in a game. That has me concerned."
Christina De Nicola is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.