Through the history NFL players have had some of the smoothest transitions to fighting in the cage. Some of the players that have gone from one to the other include:

MATT MITRIONE
Mitrione, a backup defensive lineman for the Giants, spent three years in the NFL before leaving because of injuries.

BROCK LESNAR

BRENDAN SCHAUB
Former fullback from the University of Colorado spent time with the Buffalo Bills before transitioning to MMA and ultimately retiring from the UFC.

MICHAEL WESTBROOK

The fourth pick in the 1995 draft, and eight-year NFL wideout, Westbrook turned to Brazilian Jiu Jitsu after retiring and, in 2008, he won the IBJJF World Championship as a purple belt.
Now UFC fighter and former linebacker Eryk Anders confirms football is more dangerous.

“I think football is way more dangerous than MMA,” Anders said. “Even if you get knocked out in MMA, it’s just one time. You know, you get up, you take a couple months off, you know, let your body heal. But in football, you may not get knocked out, but your constantly getting hit…especially if you play that D-line [defensive line], linebacker, running back, offensive line position. Every play that it’s a collision. You know, your body doesn’t appreciate that. You see guys that are thirty years old who their career is done, just because of all the hits, all the contact…”

Recently an NFL player was confirmed as the first living person with CTE:

CTE has been called football’s “concussion crisis,” however experts point out that CTE can develop from any repeated head injury. According to the Boston University CTE Center, “this trauma includes both concussions that cause symptoms and subconcussive hits to the head that cause no symptoms.” These subconcussive hits can include the repeated trauma the brain experiences from constant plays, hitting the turf, and tackling. Wrestlers, boxers, and military troops have also been diagnosed with the disease.