Coach Mike Shanahan said Monday that Banks was injured in last week's loss to the Detroit Lions and felt sore after practice two days later. An MRI revealed a torn meniscus, and Banks had his operation Wednesday.
The undrafted rookie from Kansas State has been a spark plug for the special teams. He ranks third in the NFL in punt returns with a 13.8-yard average and ninth in kickoff returns with a 27.3-yard average, including one touchdown.
The Redskins on Monday also placed backup safety Chris Horton on season-ending injured reserve. Horton has a high ankle sprain.