The Italian won his last 500cc title for Yamaha in 1975 and has taken extra pleasure from watching the rise of his country's new hero - Rossi being the first Italian to truly dominate the sport since Agostini's final victory at Brno in 1977.
"Valentino is the kind of talent that comes along to any sport only once in a generation - there isn't a Bobby Charlton born every year!" said 'Ago', pictured with Roberts, Rainey and other champions at Yamaha's 50th Anniversary Laguna Seca party.
"The Italian people waited 20-years for the next Agostini and Valentino has given them great happiness. I think we should thank him and thank Yamaha for a wonderful season."
With Rossi taking the title for the fifth straight season and having already signed up with Yamaha to defend it at least once more, Agostini is well aware of the extra difficulties faced when trying to stay at the top of such a demanding sport.
According to the former eight-time 500cc World Champion it is this continued desire to win that has been Rossi's most impressive asset this season.
"I think the motivation Valentino has shown this year is a demonstration of his love for the sport," he said. "The quality of the opposition is strong but Valentino has something extra. Every race is a new adventure to him and he is excited to meet each challenge. It is a fantastic achievement.
"By moving from Honda to Yamaha he made a similar switch to me, when I went from MV Augusta to Yamaha, and he has kept his motivation high. He has shown his riding talents on different machinery over the past few seasons in MotoGP and everybody around the world seems to like him.
"I am very happy because I am still very close to the people at Yamaha. I very much enjoyed riding for the factory and they are still like a family to me. This season I think Yamaha deserve just as much credit as Valentino does. The factory has done a good job, given him a good bike and he has been able to win virtually everything.
"For both of them, this year has been the best ever," Agostini concluded.