Atlanta manager Brian Snitker said Wednesday that Newcomb felt uncomfortable after a light workout.
"After working out and all yesterday, he showed effects that he hadn't felt before," Snitker said. "It's something that happens when dealing with that type of injury. Consequently, we have to make sure he's OK."
Newcomb had passed a concussion test, and the Braves were hoping he would be back in the bullpen by Wednesday.
Newcomb was injured Saturday against Philadelphia and left in the third inning. J.T. Realmuto's liner left the bat at 102 mph and hit the pitcher's head so hard that the ball bounced into the protective netting above the visitors' dugout on the third-base line.
Snitker said Newcomb likely will make a rehab appearance with Triple-A Gwinnett before rejoining the roster. Newcomb will pitch out of the bullpen when he returns. He's getting bumped out of the rotation by Dallas Keuchel, who will make his 2019 debut Friday in Washington.
The Braves recalled Huascar Ynoa from Gwinnett to take Newcomb's roster spot.