"Hunter will be on the COVID list for seven days," Reds manager David Bell said. "He could pitch any time after that as long as he gets better each day. He could start on Friday. That would be the earliest."
Greene (3-6), a 24-year-old fireballer, had three starts after returning Aug. 20 from nearly two months on the injured list with a sore hip.
His first two starts after that were rocky - 13 earned runs in a total of 6 2/3 innings - but he rebounded in his last outing on Wednesday, allowing one run and three hits through 5 1/3 as the Reds beat the Giants 4-1.
The Reds also reinstated outfielder Jake Fraley from the 10-day injured list on Friday. They also recalled right-hander Brett Kennedy from Triple-A Louisville and added newly acquired outfielders Harrison Bader and Hunter Renfroe to the expanded 28-man roster (29 for Friday's doubleheader against the visiting Chicago Cubs).
Information from Reuters was used in this report.