Newcomb, a fixture in the rotation since June 2017, couldn't hold a 4-0 lead in the Braves' victory Saturday over the New York Mets and was pulled after allowing five hits, four runs and two walks in 1⅓ innings.
Vizcaino is sidelined with inflammation in his right (throwing) shoulder. He has been injured several times in his career and felt pain while warming up Saturday night.
Newcomb, a 2014 first-round draft pick who came within one strike of a no-hitter last July, struggled badly in spring training, allowing 17 hits, 12 runs and 15 walks in 21 innings. He has struggled to consistently attack the strike zone even though the Braves are confident in his stuff.
"I told him last night when I sent him down, it's up to him," manager Brian Snitker said Sunday. "He's going to have to take stock in this and make it happen consistently. We've seen what he can do. I love what he can do, his arm, everything, but it's the consistency.
"A game like yesterday when we score those runs, you've got to go out and stretch the game. We'll see."
Touki Toussaint, who pitched six strong innings in relief of Newcomb, will take his spot in the rotation. Snitker hopes ace Mike Foltynewicz, on the injured list since the start of the season with a right elbow spur, has solid results in his third consecutive rehab appearance Monday with Gwinnett.
"That's going to be a big key," Snitker said. "If he feels good, he'll slot in sometime during the weekend."
A.J. Minter moves into the full-time role as closer with Vizcaino shut down. Vizcaino has 50 saves in his past five years with Atlanta, but he underwent elbow ligament replacement surgery in 2012 and was sidelined twice with shoulder inflammation last year.
"I think it's been bothering him," Snitker said. "Kept waiting for his velocity to spike. Thought maybe once he got in the real games it would. He got through it, but I think when he was warming up last night, I think he couldn't do it. It's a big loss for us, that's for sure."