Piastri finished 11th after a penalty for colliding with Alex Albon, while teammate Lando Norris retired with a mechanical issue as McLaren suffered its second pointless race of the year.
The team rolled the dice before the start in Montreal by fitting intermediates to both cars despite much of the field opting for slick tyres in the cold and damp conditions.
But the rain stopped before the race properly began following multiple formation laps, immediately leaving the strategy exposed.
Norris briefly made the call look inspired when he surged from third to lead into Turn 1, while Piastri slipped behind Lewis Hamilton.
However, the lack of standing water quickly forced McLaren to abandon the gamble, with Piastri pitting at the end of Lap 1 and Norris following a lap later.
McLaren were not alone in taking the risk, with both Audis, the Cadillacs and Carlos Sainz also starting on the green-walled tyres.
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Sainz was the only driver to make the strategy work by finishing the race in the points in ninth place.
Piastri admitted the conditions before the start had convinced the team the crossover point favoured intermediates.
“I mean, it was raining. And between the anthem and getting in the car it was pretty wet on the ground in all honesty,” Piastri told Sky Sports.
“There was definitely no standing water, but you could clearly tell where it was wet and where it was dry.
“And getting to the grid was not easy on slicks. Getting to full throttle was pretty tough.
“Unfortunately for us it stopped raining as the formation lap started basically.
“It’s just one of those things where if it rained a little bit more we would have looked like heroes, but it didn’t, so we looked like idiots.
“So just one of those things.”
The Australian’s race only deteriorated from there.
After falling down the order following the early stop, Piastri later locked up while attempting to pass Oliver Bearman and crashed into the side of Albon’s Williams, forcing the Thai driver into retirement.
Piastri damaged his front wing in the incident and was later handed a 10-second penalty, eventually finishing outside the points in 11th.
“I felt like I was going into the corner pretty carefully and locked the front and then that was it,” he explained.
“So, yeah, obviously not my finest moment and apologies to Alex and Williams because it was unnecessary damage for both of us. Especially for them.
“So, yeah, just one of those things.”
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Norris’ afternoon also unravelled in equally frustrating fashion.
After initially leading the race, the Briton was called into the pits on Lap 15 with McLaren suspecting a reliability issue.
Although he returned to the circuit, his race ended on Lap 40 when he stopped on track with what appeared to be a gearbox failure.
“I don’t think so. I don’t actually know,” Norris said when asked if the retirement was linked to the earlier issue.
“There was a lot of bits of metal in places they shouldn’t have been when I got out.
“So yeah, something happened that shouldn’t happen.”
Norris admitted he had also supported the decision to start on intermediates despite the gamble quickly collapsing.
“The highlight was obviously the first lap. And then I went out with a bang, literally. So just not our day,” he said.
“I guess from a decision point of view, I guess we made the decision as a team. I also pushed for it. So I kind of take the responsibility for that one.
“But also as a team, I don’t think we had the pace today anyway. Like I couldn’t even get the front tyres to get above 70 degrees. I struggled a lot.
“And I think we were just not in a good position in such cold temperatures today.
“Shame. Just not a lot of things went our way today. So we take it on the chin and we learn and we come back stronger.”
Norris also defended McLaren’s decision not to immediately abandon the strategy before the start despite the lengthy delay procedures.
“It’s tough. I still came out ahead of cars, you know, so it wasn’t like I came out last,” he said.
“The pit stop is one of the shortest of the season. Yeah, you don’t know. No one knows these things in a way.
“But that’s Formula 1 right? Sometimes it goes your way, sometimes it doesn’t.
“Sometimes you’ve got to make these decisions. And of course it worked for a lap and a half.
“If there was a safety car or things like that, it could have worked in even more to our benefit.
“But just not to be today. So we learn from the mistakes and we see what we can do better next time.”
McLaren now heads to Monaco for its 1000th Formula 1 grand prix sitting third in the constructors’ championship, 113 points behind Mercedes and 41 adrift of Ferrari.
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