Sainz qualified second for his home race, but as has been the case in previous races, Ferrari's comparatively poor race pace made him slide backwards, his Ferrari struggling with the bumpy and fast nature of the Barcelona circuit as it was outclassed by Mercedes on tyre management.
After an early first pitstop, Sainz went from the soft onto the hard tyres, which allowed Mercedes' Lewis Hamilton to breeze past him on fresher and faster mediums later on.
Team-mate George Russell also followed Hamilton through to claim a double Mercedes podium behind runaway leader Max Verstappen, and Sainz was demoted to fifth in the closing stages by the second Red Bull of Sergio Perez, who had started as far back as 11th.
While Sainz was slowly losing ground, team-mate Charles Leclerc struggled to make inroads from his pitlane start, finishing just outside the points in 11th.
Sainz said the race exposed Ferrari's propensity of chewing through its tyres, a recurring issue which was exacerbated by Barcelona's many high-speed right-handers, which were tough on the left-front corner.
"Honestly, I just spent the whole race managing tyres because we know we are very hard on them and with this high deg circuit, I just couldn't push," Sainz said.
"We know it's a weakness of our car and coming to a high deg circuit and a two-stop race, we were just managing the whole way trying to make it to the target laps of the stints and still falling short in a few of them.
"The weaknesses of our car are coming alive on a circuit like this with the high-speed corners and how hard we are on tyres. But it also shows that yesterday we must have done a pretty good lap.
"I think today was again, a bit back to where the car is at the moment in race pace, and yeah, probably this sort of track is not great for us."
Ferrari's race pace was a setback after introducing comprehensive upgrades this weekend, featuring revamped sidepods.
Sainz praised his team for fast-tracking the updates and believes Ferrari's simply hasn't been able to exploit them yet because it brought the new parts to its "weakest circuit" of 2023 so far.
"Difficult to tell," he replied when asked if the updates had their desired effect.
"I know the factory did a tremendous effort to bring them. Probably we brought them to our weakest track of this season because of the characteristics of the track. So probably we haven't seen the best of them yet.
"I still believe with the bouncing and the high-speed weakness we have we were never going to be very competitive around here.
"So, it's too early to tell but I think they did a tremendous effort to bring it, so hats off to all the factory, let's keep pushing and let's keep improving."
