The A's are awaiting further information to determine the next step for Phegley, who was originally scheduled to begin a Minor League rehab assignment on Saturday.
Phegley, 28, underwent surgery July 5 to remove a cruciate ligament cyst from the same knee.
Doolittle back on mound for Nashville
Left-hander Sean Doolittle (shoulder) made his fifth rehab appearance with Triple-A Nashville on Saturday night, one day later than planned to afford him extra rest. He worked around a double and struck out two in the outing.
The A's have been cautious with Doolittle, who delayed a potential return last week when he felt the ball wasn't coming out of his hand well. This is the second consecutive season in which he has been beset by various shoulder issues.
"We're certainly not going to put him in a position to where we're going to get him here just to get him here," A's manager Bob Melvin said. "We want to make sure he's healthy and can be effective. We're just not there yet."
Nashville takes title
Nashville clinched the Pacific Coast League American South division championship on Friday, marking the first division title for an Oakland Triple-A affiliate since the Sacramento River Cats claimed the 2012 PCL Pacific North division.
The Sounds' regular season concludes on Sept. 5, and their playoff run could impact just how many players the A's add when rosters expand.
Jane Lee is a reporter for MLB.com. Follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.