McCollum was injured in the final quarter of the Blazers' 145-117 loss to the Boston Celtics on Saturday. He was listed as probable with a rib contusion for Monday night's game against the LA Clippers but did not play in the 102-90 loss.
McCollum's condition was revealed by a CT scan, the team said.
He is averaging 20.5 points, 4.1 rebounds and 4.5 assists this season.
McCollum's injury is one of several to befall the struggling Blazers. All-Star Damian Lillard has missed four games with lower abdominal tendinopathy. Backup point guard Anfernee Simons is out with a right ankle sprain, and forward Nassir Little has missed four games with a left ankle sprain.