The Italian team started pre-season testing strongly but that did not translate into results at the start of the year, and it had to wait until the Belgian Grand Prix – after F1's summer break – for its first win.
It eventually scored just three victories, despite claiming pole nine pole positions, and was comfortably defeated by Mercedes in the drivers' and constructors' championships.
Asked where he thought Ferrari lost its fight with Mercedes this year, team principal Binotto said: "I think we lost this last year when designing that car.
"Because at the end, we've not been competitive at the start of the season. And there are reasons for it. So at the end I think it is the car project that was not good enough to start with.
"Showing our development rate [over the season], generally speaking our design was not as good as our main competitor. So I think that's it."
Ferrari should have won the second race of the year in Bahrain but Charles Leclerc was robbed of victory on just his second start for the team with a late electronic fault.
That was the first of several costly reliability problems for Ferrari, which had issues in qualifying in Austria and Germany, race-ending failures in Russia and Brazil and an engine problem in the United States.
Alongside its troubles, a stunning run of form after the summer break left it facing intense scrutiny from rivals and allegations of rule-breaking.
"It has been an intense season," said Binotto, for whom 2019 was his first year as team boss. "A lot to do. We restructured, reorganised the team.
"But I think in the meantime, we always tried to address and improve the car and I think we did it through the season, at least to some level.
"But now the season is over, and we're more looking at the next one."
Binotto was not keen to discuss the main areas Ferrari is seeking to improve with its 2020 design.
"We know where we've got at least some weaknesses, where we are weak compared to them, and are putting all our effort trying to address it" said Binotto.
"I don't know what will be the situation at the start of next season. No one can know it right now.
"We know it's a 22-race season next year. So what will be the picture at the start it still may be addressed later on.
"What's important is that the team remains united, keeps being focused, working hard, working well, and has high motivation."