Cyril Abiteboul says Renault was "missing something" as it struggled to improve the 2019 car last year.
The French works team began the year buoyed by the high profile signing of Daniel Ricciardo, and made clear progress with its engine.
But team boss Abiteboul identified the chassis as a weak point, and long-time technical director Nick Chester in December.
Not long before that, Renault had signed up Pat Fry from McLaren, but he would only be able to join the team following a period of 'gardening leave'.
On Monday, Renault announced that Fry, who has also worked at Ferrari, will start work as technical director on February 5.
Boss Abiteboul is quoted by France's L'Equipe as saying Renault needs the boost in its technical team.
"We discovered mid-season that there was a kind of limit in developing the car given the choices made in terms of the philosophy," he said.
"Despite strong investments, it was as if we were missing something from the technical leadership in terms of getting the most out of our resources."
Renault said Fry, a highly experienced engineer whose career actually started at Renault's former guise Benetton, will attend the launch of the 2020 car in Paris on February 12.