No. Mack never had any significant fumbles, unless you consider that time he was fumbling around with his bag when the cops pulled himover for speeding. The year before Byner's fumble, the defense allowed Elway to drive down the field as time expired to tie the game.
(Ironically, Byner had excellent hands. He rarely fumbled in Cleveland or Washington, and was a large part of the reason they were in a position to score right before he fumbled. Life sucks sometimes.)