The Spaniard classified fifth in the second running of the Miami Grand Prix last weekend, a result that was not affected by the imposition of a five-second penalty for pitlane speeding.
Meanwhile, team-mate Charles Leclerc - who crashed in Q3 to trigger a red flag - finished where he started in seventh. Both drivers bemoaned a lack of pace on the hard Pirelli tyre.
That lacklustre showing and shunt for Leclerc arrived despite Ferrari sporting a tweaked floor that both drivers initially reported had made the handling of the SF-23 more “benign”.
But the deficient Sunday pace has led Sainz to state the car has “zero flexibility” and that pushing in a stint harms the tyres to leave Ferrari unable to switch strategies mid-race.
Asked by Autosport whether the benefit of the revised floor had continued into the grand prix, Sainz said: “At the moment, I cannot push.
“Especially in the race, as soon as you push for one lap, you do one good fast lap and then the next lap you are 0.3s slower with the tyres that were just very, very tough.
“It means that we don’t have flexibility to push or not push and we just need to follow a certain pace to make it to the end.
“I am surprised as I thought we would be better but [Miami] showed us that we still have some work to do.”
The revised floor geometry that improves airflow to the diffuser leads a string of pre-summer break upgrades for Ferrari that will continue to arrive through Imola and Barcelona.
Recapping his Miami showing, Sainz said: “Tough. A bit surprised with how much we struggled with the hard tyres after a very strong stint on the mediums.
“We tried the undercut on Fernando [Alonso, third]. But by pushing the tyres for three or four laps for the undercut, it meant for some reason right now with our car we have zero flexibility with how much we can push and not push.
“I had to stay under the limit for the rest of the race if I wanted to make it to the end.
“With the wind conditions, how tough we are on tyres, that hard stint just became too long for us.”
He added: “We went for the undercut and it worked, it is just that with hindsight the car that we have right now, we have very little flexibility to try undercuts or overcuts.
“It is just more about how limited we are. It is tough to accept. It shows that we have a lot of work to do on tyre management and race pace.”
