Ferrari brought a comprehensive set of upgrades to its first of two home races, which it had shaken down during a filming day at Fiorano last week.
The update covered a lot of areas on the car, including the floor, the wings, sidepod and engine cover, raising hopes of the tifosi that following McLaren's leap forward in Miami with its own spec update, Ferrari could be next to challenge Red Bull at the Emilia Romagna Grand Prix.
Qualifying on Saturday burst that bubble, with Sainz and team-mate Charles Leclerc beaten by Max Verstappen and both McLaren cars.
Leclerc qualified two tenths off the Dutchman in fourth, with Sainz almost half a second in arrears in fifth.
But according to Sainz, who will move up to fourth after Oscar Piastri's impeding penalty, the new kit works completely as intended.
"It's worked exactly as we expected," the Spaniard said. "For some reason everyone expected us to be flying this weekend with the new package.
"I've been seeing numbers of our package going around that were completely out of reality.
"Nowadays, already to bring a tenth is a good job by your team. I'm not saying we've brought one, two or half a tenth, but not the numbers people were mentioning.
"With that in mind, it was always going to be difficult to take a jump this weekend because people are also upgrading their cars."
Sainz said he predicted to struggle against Piastri and Lando Norris because he expected Imola to be a "McLaren track", with Ferrari on the back foot in its first sector and through the Variante Alta chicane.
"We have come to a McLaren circuit, which is Imola, and I think it was always going to be difficult to beat McLaren around here," he explained.
Sainz was especially alarmed by Ferrari's deficit through sector one, saying it was "puzzling how we can be four tenths down in one sector."
He thought a tailwind from Tamburello to Tosa particularly hurt Ferrari's handling, although a substantial part of the gap to polesitter Verstappen's was down to the Red Bull's higher top speed, which was boosted by a tow from Haas driver Nico Hulkenberg.
Leclerc also had his own theory, suggesting to Sky Sports Italy that the Red Bull was able to take the kerbs more aggressively than the Ferraris, which have been struggling in lower-speed corners this season.
"We don't understand what we are missing there [at Tamburello]," he offered.
"We tried many things but it's always the same corners that penalise us. It's something we know and work on, it's not something we discover now.
"We know a strong point of the Red Bulls is how far they can go on the kerbs. I tried to take the kerbs, but with our car, we can't do the same thing right now."
Additional reporting by Sam Hall
