Ferrari arrived at the start of the season in Bahrain, with Charles Leclerc retiring from third place and Sainz struggling to keep up with Aston Martin's Fernando Alonso, "cooking" his tyres as he lost out on a podium.
Ahead of the Saudi Arabian Grand Prix, team boss Fred Vasseur maintained that Ferrari's main issue in Bahrain was a lack of pace rather than a return of its poor tyre management.
In Jeddah, on a much smoother and less demanding circuit, Ferrari's SF-23 challenger again struggled on Pirelli's harder C2 compound.
Putting on the hard tyre for the second stint, Sainz fell back to sixth after an unfortunately timed safety car bunched up the field.
After a strong opening stint on the soft tyres, Leclerc also struggled on the hard compound and followed his team-mate home in seventh respectively, 43 seconds behind winner Sergio Perez.
It led to Vasseur admitting afterwards that Ferrari's weakness is "clearly on the management of the different compounds".
Sainz agreed Saudi Arabia brought conclusive evidence that Ferrari's struggles cannot be considered track specific any longer.
But he revealed that the Scuderia had already identified where its deficit is coming from in Bahrain, and a good correlation in the wind tunnel has given the team a clear development focus.
"I think the last stint on the hard proves that we are not where we want to be, that we still deg more than the Mercedes, that we still deg more than the Astons and we lack a bit of race pace," Sainz said.
"I'm a bit surprised because after Friday and before the weekend, I thought that we had a chance of being the second force here in Jeddah.
"But I think that last stint on the hard proves that we still have a lot of work to do. We need to wait for the developments to come to see if we can improve that weakness.
"The thing is that the car is doing exactly the same as in the wind tunnel, so we know where the weakness is. We identified it already in Bahrain.
"We know where to develop the car, we just need time because obviously, we cannot bring the upgrades as soon as tomorrow, but I'm positive that this team is capable of bringing them early in the season.
"This could change completely our season. So heads down and work hard."
Ferrari's relatively worse race pace was highlighted by Leclerc qualifying a strong second, just 0.155s from polesitter Sergio Perez. Sainz said his tyre degradation issue on the hard tyre was even worse in traffic.
"Right now, we're not where we want to be in terms of race pace, in terms of the car balance in general and even in dirty air following," he added.
The gulf between the SF-23's peak performance and its race pace was highlighted by Leclerc qualifying a strong second just 0.155s from polesitter Perez.
Its tyre degradation issue on the hard tyre was even worse in traffic, with Sainz saying he was 'eating his tyres alive'.
"Right now, we're not where we want to be in terms of race pace, in terms of the car balance in general and even in dirty air following [another car]," Sainz added.
"If we already overheat the tyres in clean air, then imagine following, we just eat them alive. And we need clean air to produce some kind of decent lap times."
Additional reporting Matt Kew
