The Spaniard was unable to match the speed of team-mate Charles Leclerc from the first lap at Zandvoort, and eventually fell back into the pack of pursuing cars.
But despite valiantly trying to hold off Fernando Alonso for sixth place, he eventually lost out to a DRS move down the start/finish straight at the start of the final lap.
Speaking after the race, Sainz said that there was no obvious explanation for why he was so slow and said only an investigation back at Ferrari’s factory this week will get to the bottom of it.
“I cannot explain it obviously,” he said.
“The whole race was a bit of a pain for me, and I was just very slow: sliding around, and degrading the tyres.
“I lost the feeling that I had with the car on Friday on Saturday. And, to be honest, on the laps to the grid, I already noticed something that I was not really happy with, and the balance was quite off.
“I basically struggled quite a lot the whole day, and it’s something that we are going to look into in the next few days to try to find out exactly what happened.”
Marshals remove the damaged car of Carlos Sainz Jr., Ferrari SF21
Photo by: Andy Hone / Motorsport Images
Sainz’s weekend was not ideal, after he crashed heavily in final free practice on Saturday morning and damaged his car.
While Ferrari was able to repair it fully, the team says it cannot rule out something being a legacy of that incident.
Team boss Mattia Binotto said: “We don’t know yet what really was the issue, so we will look at everything.
“We will look at the car, in terms of assembly. We will look at the setup, we will look at the car balance. At the moment we do not have any answer.
“From the crash, I don’t think that there is any consequence of it, even though Carlos did not have useful track time in the morning to try to understand a bit more the car.
“I think we cannot exclude a mistake at the moment, so we will look carefully at all the aspects.
“I am sure we will come out with a conclusion that will make it more comfortable for the next events.”
Sainz added: “It might either be one thing or it could be a lot of small things. I prefer not to jump into conclusions, particularly because I don’t know exactly.
“We simply haven’t had a look at the data, so it’s going to take a few days.
“I’m sure we will find something and we will come back stronger. It might just be that it was nothing and I need to change a few things. But it is true that it has been very strange and we’re going to work hard on understanding it.”