He suffered a partial dislocation and a trainer appeared to have popped the shoulder back into place.
Wednesday's injury came when McCarthy stretched out his left arm to collect a toss from first baseman Freddie Freeman on a groundout by Brian Goodwin. McCarthy immediately grabbed at his shoulder and a trainer came out to attend to him. McCarthy recorded the out, which was the last of the bottom of the fifth inning.
The Braves said McCarthy, who gave up one run on four hits against Washington, is day-to-day. The pitcher indicated after the game that he suffered a similar injury in the past.
"Last year was the first one, so that just feels like a gunshot, 'what the hell's going on,'" he said. "And then the second one, I now know what it is, so it's less of a shock and then it's 'breathe, calm down, let's get this back in.'"
Left-hander Sam Freeman replaced McCarthy on the mound to start the bottom of the sixth. The Braves went on to beat the Nationals 5-3 in 12 innings.
Last season with the Los Angeles Dodgers, McCarthy dislocated his left shoulder during a kettlebell exercise and landed on the disabled list.
The Associated Press contributed to this report.