Spaniard Sainz, now at Williams, clashed with Piastri while the Australian was trying to lap him and retain the lead after his second pit-stop.
The lost seconds allowed Piastri's teammate and world champion Lando Norris to take the lead when he pitted several laps later after building up a crucial advantage, going on to take the team's first win of the Formula One season.
Piastri ultimately retired with a broken gearbox while in second place with McLaren heading for a one-two.
"My understanding is that the blue (warning) panels were not working," Stella told reporters. "So perhaps today was more difficult than normal for the backmarkers to appreciate what was happening in their mirrors.
"Carlos is a very fair, very correct driver. So I'm sure that was absolutely unintentional but a bit of a shame for Oscar.
"It's absolutely understandable that when you lead the race for almost half (the distance), and then you see yourself losing the lead because of an incident with a backmarker, then you may have a moment and make a comment in anger."
Radio rage
Piastri, who had overtaken Norris on the opening lap, was scathing about Sainz in his radio messages and afterwards.
"He was fighting (Aston Martin's) Fernando (Alonso) for last place like it was the world championship and cost me the lead of the race," the Australian told reporters.
"He's quite critical of others, and others have given him some stick before for being frustrating on track, and when you go and do something like that, I think maybe you should look in the mirror a bit.
"I don't really care if he didn't see me. The fact that he didn't, or no-one told him, or there was a lack of complete awareness, is unacceptable."
Piastri, seventh in the championship and 36 points behind Norris, had appeared to blame McLaren initially for bringing him out of his pitstop into traffic.
However, he later recognised that McLaren could not be blamed for strategy decisions that did not factor in being taken out by a slower driver he was trying to lap.
His radio reaction at the time was more explosive, and punctuated by expletives.
Sainz, who was handed a five-second penalty, said it had been impossible to avoid the collision and dismissed it as "a racing thing that sometimes happens".
"We didn't have blue lights on the steering which normally, when we are in fights like that, where we are fighting for position, helps us to understand whether we are getting lapped or not," he said.