Renault ousted Nico Hulkenberg because he is too "pessimistic" and "negative".
German Hulkenberg, linked with moves to Alfa Romeo or Haas, is currently looking for a new home for next year.
"We don't want pessimistic drivers, which is why we opted for Ocon," Prost told Sky Italia.
"He (Ocon) is very motivated after being out for a year. Nico is quite negative, but he's also right about certain things. But we have to look forward and a driver's negativity doesn't work for us, especially in a difficult season."
Prost agrees with Hulkenberg that the Renault chassis needs to be improved, but he calls the team's 2019 engine "excellent".
However, he admits that 2020 could be another difficult year for Renault.
"I'll be honest, due to the change in regulations in 2021, it makes no sense that we continue to develop a lot. We hope to have a high level in 2021," said Prost.
"We will have to invest a lot in the development of a good car to be in front again," he added.