Terrible news. Saw this on twitter from Jordan Wagner. 

"The career-ending issue came down to a diagnosis of arthritis in Johnson’s right knee, which initially caused him problems when he played through a torn meniscus as a senior at Central."

Here is the link (sorry its from a mobile phone):

http://www.gwinnettprepsports.com/news/2015/jan/10/with-ohio-state-in-championship-game-central8217s/?sports