Lamb injured his head (likely concussion) in November, causing him to miss one game. Having one recent head injury generally makes him somewhat more susceptible to another, and damage to the brain is cumulative. However, his brief time away suggests relatively mild severity. He also injured his shoulder in 2017, but did not miss games or undergo surgery as a result. Concern level is therefore low.
Image Source: Nicole Cordeiro, CC-BY-2.0