Complex sentence: He played professionally in the National Football League (NFL) as a running back for 11 seasons, with the Buffalo Bills from 1969 to 1977 and with the San Francisco 49ers from 1978 to 1979.
Split into simpler sentences: He played professionally in the NFL as a running back for 11 seasons, primarily with the Buffalo Bills from 1969 to 1977. He also played for the San Francisco 49ers from 1978 to 1979.