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.
With Colin Kaepernick unable to play because of shoulder tightness, the 49ers had Lewis succeed starter Blaine Gabbert for their fourth series, and Lewis passed for a team-high 97 yards (12-of-21).
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.