Quarterback Thad Lewis’ bid to serve as a 49ers backup is over because of a knee injury, a league source confirmed Monday.
Lewis suffered a torn anterior cruciate ligament during the third quarter of Sunday’s 24-13, exhibition-opening loss to the Houston Texans.
Lewis, one of the 49ers’ few free-agent acquisitions, figured to battle rookie Jeff Driskel for a No. 3 role this season, assuming Gabbert and Kaepernick would fill the top two slots. Driskel was 4-of-12 for 20 yards in his pro debut.