"I think it was a concussion," Shanahan said. "So, I'm not sure. I know they told me he was out so I don't get much information on it. I just know that it was a concussion and I'll find out when I go back in. I'm sure he'll be in the (concussion) protocol."
The good news for Juszczyk is that he will have 10 days to get healthy, which should be enough time to make sure he is available for the 49ers' October 1 matchup against the Arizona Cardinals.
The loss of Juszczyk changed the way Shanahan was able to call the game.
"We lost our fullback in the second half, which changes some stuff up too," Shanahan explained. "The type of runs you can run. When you get into the red zone, especially inside the five-yard line, there's lots of fronts and coverages people can play and send all-out blitzes and stuff. If you don't have a fullback, then you're going to be outnumbered or if you don't use your quarterback as a runner, you're going to be outnumbered.
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"They put you in a bind in some spots. They've got to force you to pass and you still aren't going to do it every time. You've got to mix it up based on the looks you get."
Juszczyk, 26, was among the 49ers' hauls at the start of free agency. He signed a four-year contract with San Francisco that will earn him an average of $5.25 million per year.
Juszczyk was a fourth-round selection out of Harvard by Baltimore Ravens in the 2013 NFL Draft. He was selected to the Pro Bowl last season and was a 2016 First-Team All-Pro. Juszczyk has been a capable receiver as well. He has 97 catches for 769 yards and five touchdowns since 2014.
Against the Rams, Juszczyk had one reception for 34 yards before leaving the game. Prior to that, he had two receptions for 17 yards with the 49ers this season.
Jaquiski Tartt and Brock Coyle also suffered concussions against the Rams and left the game. Tartt's head injury came after he was accidentally kicked in the back of the head by teammate and defensive lineman Earl Mitchell during the third quarter.
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