The 32-year-old Diggs, who is in his 11th season out of Ohio State, joined the Rams in the offseason as a free agent after four seasons with the Carolina Panthers and solidified the strongside linebacker spot.
"I thought he played one of his better games actually yesterday, he really did," Spagnuolo said. "I think he had a couple tackles for losses. And he's just been a real strong, veteran presence in there. Not real vocal, but I think the guys have a lot of respect for him because of where he's been, what he's done, what he's about, how he works and hopefully he can get through this surgery this week and we can just have his presence around here. I think that would be good. But I think that's unfortunate."
In 12 games, all starts, Diggs had 36 combined tackles, two passes defended and one force fumble. He ranked seventh on the team in tackles. He will become the 11th Rams player to go on injured reserve.
The Rams have struggled at weakside linebacker spot this season, starting four different players — Larry Grant, Bryan Kehl, Chris Chamberlain and David Vobora — there in the first 12 games. Now they'll have to figure out both outside linebacker spots.
"I mean we've got guys that have played, I mean, Dave Vobora's played over there, Larry's played over there," Spagnuolo said. "We have guys. We'll certainly have to get another linebacker in here because that only leaves us with five really on the 53 (man roster). So we've got some decisions."
Linebacker Curtis Johnson, a 25-year-old from Clark Atlanta University, could be signed off the practice squad to take Diggs' roster spot.