 Complete the following phrase:  The following season, Riva made his Serie A debut for Cagliari on 13 September, 1964, in a 21 loss against Roma, and also helped the club avoid relegation, leading the team to a comfortable sixth-place finish in the table, scoring an encouraging 9 goals in 32 appearances at the age of 20. This was followed by eleventh, sixth, and ninth place finishes in the league between 1965 and 1968, while Riva also finished as the leagues top-scorer for the first time during the 