As refered to the websitehttp://www.buddhanet.net/e-learning/history/db_01.htm
If Sariputra has attained Nirvana, why does the Buddha say this?
Buddha said: "Sariputra! I predict one day you will become a Buddha by the name of Pamaprabha. You will come to this world again to save all living beings and achieve the highest state of Buddhahood."
The Buddha describes the arahant as having transcended ‘the round of birth and death, they have destroyed the taints, lived the holy life, done what had to be done, laid down the burden, reached the ultimate goal, destroyed the fetters and become completely free, liberated through final knowledge’ (Majjhima Nikaya 1. 141).