Mickelson, who moved up to No. 2 in the world ranking after his victory in The Players Championship, believes he injured his wrist while chipping out of the deep rough at Oakmont earlier this week during practice for the U.S. Open.
He withdrew from the Memorial after 11 holes Thursday.
A specialist in San Diego found no break, but a statement from his management said Mickelson will take anti-inflammatory medication for the next few days and begin therapy on Monday.
Mickelson entered the Stanford St. Jude Championship next week in Memphis, but it didn't seem likely he would play.
'My intention still is to play Memphis, but at this point, realistically the chances are probably 50-50,' Mickelson said in the statement. 'I don't want to jeopardize the opportunity to compete in the U.S. Open.'
The U.S. Open starts June 14 at Oakmont.
Mickelson is known to spend as many as eight hours on the course while preparing for a major, setting tiny flags around various spots on the green and chipping from every conceivable angle.