Niemann was 1-4 with a 5.74 ERA in six starts. The right-hander last pitched Wednesday against the Toronto Blue Jays, allowing three runs in four innings.
The 25-year-old Guyer was in the starting lineup Friday night against the Baltimore Orioles. It will be his major league debut.
Guyer hit .356 with six homers at Durham. He led the International League in hits (37) and ranked fourth in runs (21).
At spring training this year, Guyer batted .100 in seven games.