Also found this article that paints a slightly more positive picture in terms of recovery. I've edited the excerpt to only include the pertinent info. I also don't know if Clark has an "upper-level" cervical disc herniation (CDH), or "lower level" issue. You can check out the full article here:
https://www.newswise.com/articles/nfl-players-get-back-in-the-game-after-upper-spine-surgery
Newswise — October 4, 2016 – The majority of U.S. National Football League (NFL) players who undergo surgery for a herniated disc in the upper (cervical) spine are able to resume their careers and perform at a high level, suggests a study in
Spine, published by
Wolters Kluwer.
Even players with "upper-level" cervical disc herniation (CDH) have a high return rate and can return to play with similar performance outcomes compared to their “lower-level” counterparts, according to the analysis by Dr. Harry T. Mai of the Department of Orthopaedic surgery at the Northwestern Feinberg School of Medicine, Chicago, and colleagues. The results may help guide decisions about surgery and the likelihood of returning to competition for players with these career-threatening injuries.
Good Chance of Continuing NFL Career after CDH Surgery The researchers identified 53 NFL players who underwent surgery for a herniated disc in the cervical spine between 1979 and 2013. Forty players had confirmed data on the level of the disc injury: 15 had "upper level" CDH (involving the uppermost vertebrae of the neck) while 25 had "lower level" injuries. Forty-five percent of the players were defensive backs and linebackers.
Rates of successful return to play after CDH surgery were analyzed.
Most players—67 percent of those with upper-level CDH and 72 percent with lower-level injuries—were able to return to play after surgery and rehabilitation, with no significant differences between the two groups. In both groups, recovery time was about nine months. On average, players continued playing for about 44 games and three years after CDH surgery [...]
In 34 players, surgery consisted of
spinal fusion (a procedure called anterior cervical discectomy and fusion, or ACDF). Four players developed degenerative disease at an adjacent vertebral level, requiring additional surgery. While this is a serious complication, it appeared to be no more frequent than in non-athletes undergoing ACDF.
Cervical disc herniation is common among NFL and other elite athletes in contact sports and appears to be more common in upper level discs than the general population. [...] For football players, the expected outcomes after CDH surgery—including the player's ability to continue to compete at a high level—are unclear. That's especially true for patients with upper-level CDH injuries.
The results suggest that most NFL players who undergo cervical spinal surgeries for CDH are able to resume their careers. Return to play, and level of performance after returning to competition, appear similar for players with upper-level versus lower-level cervical injuries. The authors note some important limitations of the study data, including potential "selection bias toward more prominent athletes."