Speaking at the Canadian Grand Prix, Stella praised the 25-year-old Piastri's performances and mindset, adding speculation around his career was just 'silly season' chatter typical of the rumours surrounding drivers.
"When we think about Oscar, we couldn't be happier. I think we are seeing the best Oscar in the cockpit and also a happy Oscar and the best version of himself outside the cockpit," Stella told reporters.
"Great dynamic and relationship with (teammate) Lando (Norris) and I think the team is in its strongest shape since I've been team principal.
"So definitely it's very clear the direction for maximum stability at McLaren," he said.
Piastri struggled in the opening rounds of the season in Australia and China, failing to even start either race. However, he went on to achieve two podium finishes, taking second place in Japan and third spot in Miami.
Online speculation has linked Piastri to Red Bull as a potential successor to Max Verstappen, were the four-times champion to leave, but the team's boss Laurent Mekies was also keen to play that down.
"Honestly, it's not so dissimilar to Andrea's comments, in the way that we have a very, very strong driver lineup, and the driver choice is not a question that we are asking ourselves right now," said the Frenchman.
"We have Max, nobody needs to present Max. He is pulling us up through a fairly complicated start of the season."
"Isack (Hadjar) is doing a very, very good job. As far as we are concerned, we are super happy with the two guys behind the wheels."
Piastri, meanwhile, drove his car to fourth place in the grid for Saturday's sprint race at the Canadian Grand Prix.
George Russell beat Formula One championship leader Kimi Antonelli to take pole position, with the British driver clocking one minute 12.956 seconds to beat his Mercedes teammate by a mere 0.068 seconds.
Defending F1 champion Norris was third, 0.315 seconds off the pace, while Piastri was followed by Ferrari's Lewis Hamilton and Charles Leclerc.
Verstappen and teammate Isack Hadjar were seventh and eighth respectively.
This is a second sprint pole for Russell, who also started in first on his way to victory at the Chinese GP sprint race in March.
"This feels great after a tough Miami but I never doubted myself. I know what I can do and this is an amazing circuit with high grip," Russell said.
"The team have done a great job to bring things forward and there is now a big focus for tomorrow. We are making baby steps."
The qualifying had a major delay due to a red flag in the first part after Fernando Alonso crashed his Aston Martin into the wall.
Qualifying for Sunday's main race will follow the sprint race.
Agencies
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