Third cornerback Chris Culliver sustained a torn left anterior cruciate ligament, an MRI examination revealed Thursday, two sources confirmed to CSNBayArea.com.
Culliver went down during a non-contact punt coverage drill. He was carted off the field in tears and underwent the examination at Stanford Hospital. The NFL Network first reported the torn ACL Thursday evening.
Culliver's absence leaves a hole in the 49ers' pass defense behind starting cornerbacks Carlos Rogers and Tarell Brown.
The 49ers also return Tramaine Brock and Perrish Cox from last year's team. Also, veteran Nnamdi Asomugha will continue to compete for playing time.
Culliver will be placed on injured reserve, a source said. He will make his entire scheduled salary of $555,000. Culliver is signed through the 2014 season.