There have been men who earned PhDs in mathematics during or after NFL careers--quite recently HN had links to an obituary of Frank Ryan, who led the Cleveland Browns to their last NFL championship, and who taught at Case. More recently, John Urschel, a lineman for the Baltimore Ravens earned a Ph.D. and according to Wikipedia teaches at MIT.

I have the impression that college football was much less of a full time job in Ryan's day, but Urschel graduated from Penn State in 2012.