Poindexter injured his knee on special teams in the second half of Monday night’s preseason game against the Broncos.
He played one snap on offense and four on special teams Monday. In the opener, he caught two passes for 12 yards against the Cowboys.
Poindexter appeared a candidate for the 49ers’ practice squad.
The 49ers signed receiver Chris Thompson to take Poindexter’s spot on the roster, Shanahan said.