Dufek was signed as, more or less, an afterthought when Jim Kelly, the quarterback the Bills had drafted in the first round of the 1983 draft, opted to not sign with the Bills and jumped to the United States Football League. By the 1984 season, Dufek had risen to the second-string quarterback position, behind an aging and fading Joe Ferguson. Ferguson's performance continued to deteriorate, and on September 30, after 107 consecutive games started, Ferguson was benched, and Dufek received his first game as starter. Dufek and Ferguson combined for a 2–14 record in 1984; Ferguson was released at the end of the season, while Dufek made it through part of 1985 (not starting any games that season) before being cut.
Dufek finished the 1985 season and his professional career with the San Diego Chargers. With the USFL folding as a result of a failed antitrust lawsuit, Dufek was one of the casualties of the tighter job market for professional football players, never playing professional football again and declining to cross picket lines in the 1987 strike.