Simpson had his finest season in 1971–1972, in which he averaged 27.4 points, 4.7 rebounds, and 3.1 assists. After the ABA–NBA merger in 1976, Simpson joined the Detroit Pistons, but his level of production dropped significantly, from 18 points per game in 1975–1976 to 11 points per game in 1976–1977, and after one-and-a-half seasons, he returned to the Denver Nuggets, now one of the ABA teams who had joined the NBA. However, he averaged only 5.5 points during his second tenure with the Nuggets, and after two more seasons as a member of the Philadelphia 76ers and New Jersey Nets, he retired in 1980.