The Spaniard lined up sixth on the grid following Charles Leclerc's formation lap retirement but lost out at the getaway to Sergio Perez and George Russell to slip to eighth by the time the race was red-flagged for a hefty Turn 1 shunt involving Alex Albon and Kevin Magnussen.
Sainz then radioed to say his team needed to “bin” the clutch as he bemoaned start problems all weekend. He would later complain about the left gear shift paddle “sticking”.
The Singapore GP winner confirmed it was a faulty part rather than a calibration issue.
Asked by Autosport to explain his poor getaway, Sainz said: “[It was] the same as [Saturday] and the same as every single start that I've done this weekend.
“I’m very disappointed because it obviously cost us probably some points over the weekend.
“We had a problem since the beginning of practice. We weren’t able to really tune it and adapt it to the track and we have been struggling a lot with getaways and it's such a shame.”
Sainz added that launches had been a strength of Ferrari all season but realised quickly that the team was “struggling a lot”.
Sainz’s own reliability scares came after second-starting team-mate Charles Leclerc crashed out on the formation lap due to an electronics failure that killed the engine hydraulics and power steering.
“Thank God nothing happened to me,” said Sainz. “It's a shame because I think with Charles in the race it would have been a great opportunity to take away points from Mercedes.
“But also, the car pace wasn't great either. So, Aston [Martin], McLaren and Red Bull were up in front, and we couldn't fight them really much. A missed opportunity, I think.”
Ferrari team boss Fred Vasseur reckoned the clutch issues for Sainz were not as widespread as his driver had made out, saying that the launch at the restart after the red flag was spot on.
The French engineer explained: “On the first start, he had a poor start. The second one went well. We need to understand exactly what happened on the first one, but it was not the best.
“We are struggling a little bit with the starts. Obviously, if you have a look, it’s marginal but the second one went pretty well.
“[Interlagos] is also a bit more difficult because you’re starting with your position a little bit on a hill.”
Vasseur reckoned that Sainz’s gearshift issue had only lasted for one corner.
Additional reporting by Adam Cooper