With light rain falling in cold conditions in the build-up to Sunday's race in Montreal, both Piastri and Lando Norris were among seven drivers to fit intermediates for the start - Audi's Nico Hulkenberg and Gabriel Bortoleto, Williams' Carlos Sainz, plus Cadillac's Sergio Perez and Valtteri Bottas were the others.
From third, Norris actually took the lead from the two Mercedes but pitted at the end of the second lap as his intermediates began to overheat.
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Piastri said over the radio that McLaren had "made a mistake" as a problem for Racing Bulls' Arvid Lindblad led to there being two extra formation laps, delaying the start around seven minutes later than scheduled.
"It was raining and between the anthem and getting in the car, it was pretty wet on the ground. You could clearly tell where it was wet and dry," Piastri told Sky Sports F1.
"Getting to the grid on slicks was not easy, getting to full throttle was tough. Unfortunately for us, it stopped raining. If it rained a little bit more we would have looked like heroes. It didn't, so we looked like idiots."
However, Stella thinks had the rain "lasted for a few more minutes and the start happened at the right time", the drivers on dry tyres would have struggled.
"At the time that you had to make a decision as to what tyres to fit, and with not necessarily a clear idea as to when the rain would stop, the track was greasy and the right tyre at that time was the intermediate tyre," he said.
"I was really interested in seeing a race start at the time when the race should have started because I'm not sure how long it took for a double extra formation lap, but definitely if you look at the pit lane, it went from being dark grey to grey, like dry.
"I think we always have to be a bit careful in judging decisions simply from the outcome. I think you have to judge decisions at the time that they need to be made.
"The rain stopped pretty much after the five-minute signal, and then the double extra formation lap added a clear penalty to starting on inters."
Are McLaren vulnerable in cold conditions?
Norris and Piastri locked out the second row in Sprint Qualifying and Qualifying in Montreal behind the dominant upgraded Mercedes.
McLaren also brought new parts to Canada but did not run their new front wing after testing it in Friday's only practice session.
Norris managed to split the Mercedes in Saturday's Sprint and got back into the points-paying positions on Sunday after pitting back onto the dry tyres.
However, he was forced to pit again to clean the radiators, then suffered a gearbox failure which caused his second retirement of the season.
"In the cold conditions we couldn't get the fronts up to temperature, which made the car hard to handle, but even so I think we were on for decent points," said Norris.
"There are still plenty of positives to take from the weekend. Our pace was strong, we fought the Mercedes, and the support from the fans was brilliant.
"We'll investigate the failure, work on tyre warm-up in cooler conditions, and come back stronger. We learn from the calls we made and aim to turn that speed into a clean finish and points next time."
Piastri finished 11th after damaging his car - and receiving a 10-second penalty - following a collision with Williams' Alex Albon as he sought to fight back through the field.
Stella believes, even had McLaren started the race on dry tyres, neither Norris or Piastri could have challenged for the podium.
"I don't think we exhibited any competitive pace at the level that we have exhibited up until Sunday. I can only point this towards the lack of tyre temperature," he said.
"The drivers kept locking tyres and going long in corners and just not getting the front tyres to work at all.
"I would say that even in a normal race, in a race in which we had no issues, looking at the pace of the other cars that were contending for the podium."
Next up is the start of Formula 1's European summer swing, with the Monaco Grand Prix the first of six races in eight weeks. Watch live on Sky Sports F1 from June 5-7. Stream Sky Sports with NOW - no contract, cancel anytime
