Lando Norris took his maiden win in Miami as McLaren rolled out a significant 10-part upgrade package.
While Red Bull's polesitter Max Verstappen struggled with floor damage, Norris was handed a free pitstop as he had stayed out until a mid-race safety car was called.
But even before his lucky break Norris displayed searing pace on the medium tyres, and on the hard-tyred restart he comfortably held his lead and drove away from Verstappen to win by seven seconds, with the Dutchman admitting McLaren's pace looked "insane".
Piastri was also rapid before contact with Carlos Sainz knocked him back, in a car which unlike Norris' MCL38 didn't benefit from the full suite of upgrades yet.
And while emotions ran high at McLaren following Norris' breakthrough win, Stella said he wasn't too sure that Miami's Sunday picture was representative for the races to come.
"Yes, we improved the car. We know in the numbers how much we improved it and its material, you should see it in laptime," Stella said.
"But for Red Bull, this race was a little bit of a struggle. Is the pace we saw a representation of the future? I'm not going to put my signature on this statement.
"In Japan, we were six-tenths away in qualifying off Red Bull. In China again we were a few tenths away, so it would be completely unrealistic to think that what we have seen here is the updated picture of competitiveness.
"I think here Red Bull didn't get the most out of their package."
With Piastri also getting the full spec in Imola, which includes a revised floor, Stella says the team's Miami success should be seen as a sign that its work in the factory is paying off.
It will now start pulling together its next upgrade package for later this year, which Stella believes is necessary to pose a consistent challenge to Red Bull.
"We take this positive, we take this encouragement," he added. "If anything it's even more energy to develop possibly even faster than what we are doing.
"But in my view, if you want to fight consistently Red Bull, we need to deliver another package like the one we have delivered here."
