Although Matt Mitrione proved that former NFL players can make the transition to MMA, it is going to be quite difficult to succeed at a high level if one does not focus completely on training. I'm not counting Herschel Walker because he's retired, only 2-0 and both of his opponents were hand-picked for him.
Tom Zbikowski, on the other hand, does have actual boxing experience, but that still doesn't mean he should make the transition to MMA.
No one wants to see these players get injured and their lack of fighting experience will all but ensure that they do. As Marcus "Big Baby" Jones showed, just because someone is athletic and strong doesn't mean they can take a punch.