SANTA CLARA, Calif. -- Another day, another potentially significant injury to consider.San Francisco 49ers cornerback Chris Culliver was running downfield during practice Thursday when he fell to the ground and clutched his knee. The 49ers scheduled an MRI exam. They did not yet know the extent of the damage. They did not speculate.Culliver was covering a punt during a non-contact portion of practice. There was no collision of significance. Culliver initially sat on the ground and extended his left leg a couple times.