Jordan Mason was a sensational replacement for the injured Christian McCaffrey.

Now the San Francisco 49ers are hoping rookie Isaac Guerendo can fill in as admirably after Mason was injured in the second quarter against the Seattle Seahawks on Thursday night. Mason headed to the locker room for X-rays and is questionable to return due to an injured shoulder.

By midway through the second quarter, Mason had already rushed for 65 yards on eight carries, bringing his NFL-leading season rushing total to 601 yards.

Now the 49ers will move on, with Guerendo likely leading the rushing attack. The question is how long the rookie will be called upon to do so.