Foster is the fifth 49ers rookie to win the award, according to the Sacramento Bee. The previous four were Andre Carter, Patrick Willis, Aldon Smith, and Chris Borland.
"When we drafted him, I thought if he could stay healthy, he'd be the Rookie of the Year," 49ers defensive coordinator Robert Saleh said on Thursday. "No question the defense is built for linebackers to run and hit and get tackles, so for him to gobble them up the way he has, it's not a surprise.
"He's a very talented and instinctual player. He's very explosive, so the sky's the limit. He hasn't even scratched the surface too. If he continues going the way he has, it's only going to get better."
49ers head coach Kyle Shanahan joined KNBR on Friday morning and discussed his rookie linebacker earning the NFL honor.
"I was happy for Reuben," Shanahan said on the "Murph & Mac" show. "He's been battling an injury all year, especially since that first game. Came in with the shoulder injury, so he missed OTAs, and we got him back in training camp. Then he got hurt the first game against Carolina. When he came back, he got reinjured. So it's been lingering for him all year. I also know he's not fully healthy right now. But what has been good is he's been able to stay out there for three full games.
"When Reuben is out there, I think everyone can see he is a very good player but by no means has he fully arrived. He's still got a long way to go, still can get a lot better. I've got very high expectations for Reuben and I'm glad he got the recognition this week, which I think he deserved, but I also plan on Reuben continuing to get better. I hope he'll be better this week versus Chicago than he was last week and I hope we keep continuing that throughout the year."
Shanahan also discussed quarterback Jimmy Garoppolo's preparation to start on Sunday against the Chicago Bears and the 49ers having fun during their final five games of the season.
You can listen to the entire interview with Shanahan below.
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