Sainz was incensed when Ferrari called him into the pits on lap 33, one lap after the Alpine of Esteban Ocon that Sainz was shadowing during the entire first stint.
Despite a slow stop from Alpine, Sainz didn't manage to overcut the Frenchman and he felt Ferrari should have been more patient as he had more pace in hand on his tyres, which he had been able to save while being stuck behind the Frenchman.
“What the f***! This is exactly what I talked about,” he raged on the radio, and when told the aim of the pitstop was to cover off Lewis Hamilton behind, he replied: “I don’t care about Hamilton. This is weak."
Sainz later showed remorse for his outburst, saying he "shouldn't have showed it on the radio.
"Obviously, I was very quick on the in-lap and felt like I still had a lot more lap time to come in clean air.
“I had been doing all that management. To suddenly pit, it left me frustrated."
Team boss Vasseur defended the timing of the pitstop, because the team felt the threat from Hamilton's Mercedes was simply too big to leave the Spaniard out.
"I think it was a good strategy because when we asked him to pit it was to avoid losing a position against Hamilton," said Vasseur.
"Positions are key on this track. It would have been better to extend if we were not at risk from Hamilton, but in this situation I think it was the good call."
Sainz had been visibly frustrated at being stuck behind Ocon, making ambitious overtaking attempts at the chicane which left him with front wing damage, and which Vasseur labelled as "a bit optimistic and on the edge."
Ferrari also decided against extending its first stints hoping to cover for any rain, like race winner Verstappen, Fernando Alonso and George Russell.
"At one stage we had to cover the other ones on track to avoid losing position and it's always easy to redo the race after 5:30 and say 'okay, if you extend the stint then you can change the tyres when it's raining'," Vasseur added.
"But we didn't know at this stage, and we were in the situation to lose positions compared to our competitors."
Ferrari lost out a second time when the rain did come in the final third of the race. Ocon and both Mercedes cars of Hamilton and Russell came into the pits on lap 55 as the rain intensified.
The Ferraris opted to stay out one more lap on slicks, with Sainz spinning off at Mirabeau before both he and Leclerc then gingerly shuffled past the slippery swimming pool towards the pits.
That dropped Leclerc and Sainz behind Hamilton and to add insult to injury, a double stack behind Leclerc meant Sainz lost another position to Alpine's Pierre Gasly.
The pair eventually crossed the line in sixth and eighth respectively, from fourth and sixth on the grid.
Vasseur admitted the call to stay out on slicks was a gamble, one also taken by other teams on a half-wet track, but that it was "worth taking" to try and recover a podium on a circuit where overtaking is impossible.
"We were expecting that the track wouldn't be so wet and that if you don't have to put the wet [tyres], that we could have finished on the podium," the Frenchman explained.
"It was the gamble for us, the risk was not too high because we didn't lose position. But it is like it is.
"We knew perfectly on the grid that if we want to achieve something, we have to take risks. I'm not disappointed at all with the risks that we took."
