"We were just too late," he told Germany's Auto Motor und Sport.
"We have seen in recent years how success is made -- Red Bull had a plan and was world champion four times. Mercedes had a plan and is now far ahead at the front.
"Renault needs a plan for 2015," Prost insisted.
However, the 59-year-old clarified that Renault cannot just give up on 2014 and focus exclusively on re-homologating a much better 'power unit' for next year.
"We have to improve continuously," said Prost.
"Nobody can say where Renault is today compared to Mercedes -- Renault made a big leap in the Spanish grand prix," he claimed.
"Mercedes is still ahead, but many have misunderstood that the jump really came from their car."
Renault, supplying four teams in F1 including reigning world champions Red Bull, says it is now approaching the full potential of its current V6 unit.
"We are still with our plan of getting 100 per cent from the engine in Canada," said Rene Taffin, referring to the next grand prix in two weeks.
Red Bull, however, is not so sure.
Toto Wolff, the Mercedes team boss, said on Thursday that a Mercedes-powered car got up to 347kph on the Barcelona straight recently.
"So in Monza I don't know what we will be seeing, but 380 maybe?," he said.
Red Bull counterpart Christian Horner replied: "I don't think we'll be seeing 380 from our car in Monza."