Forbort suffered a lower-body injury just more than three minutes into Monday's game against Colorado and reportedly left Staples Center on crutches.
"We'll just see how he responds the next few days here, but neither one will be available for [Thursday]," coach John Stevens said.
Muzzin and Forbort are two of the Kings' top four defensemen in ice time. Forbort is tied for 14th in the NHL in blocked shots. Muzzin has missed three games with an upper-body injury and has been skating.
Alex Iafallo appears to be progressing with an upper-body injury and could join the team for practice Wednesday.