Both Stella and Mercedes F1 boss Toto Wolff said it was not yet clear whether the oil leak was caused by a chassis or engine problem.
Stella also praised Piastri for the calm way he had built towards pole and victory after being a step behind Norris throughout the practice sessions.
“I would say that Oscar’s weekend has been a characteristic weekend for Oscar because we have seen the characteristic of building up through the weekend in practice, cashing in the learning,” Stella said.
“We have seen the delivery at the moment that counts, which is qualifying.
“And then we have seen that when it comes to race, he is characteristically quite comfortable, keeps the situations under control, pretty sharp in execution. And while he does so, he remains always calm and lucid.
“So this is one of the, let me say, Oscar-like weekends. It proves his maturity and the level of his race-craft as a Formula 1 driver.”
Norris’ situation is reminiscent of the one that faced Lewis Hamilton when he was trying to recover a gap to Mercedes team-mate Nico Rosberg in 2016.
Hamilton was behind Rosberg in the championship as a consequence of a series of reliability problems. He was poised to reclaim the lead with a dominant victory in Malaysia towards the end of the year, only for his engine to fail.
That left Hamilton needing to close a 23-point gap to Rosberg in the final five races.
The German put himself in a strong position with a win in Japan the following weekend, and Hamilton fell just short despite winning the final four races in a row.
Wolff said on Sunday that the Malaysia retirement had created strains in his relationship with Hamilton.
“Super difficult,” Wolff said. “Because you’re letting a driver down. Now, you could say maybe it’s a long season and that was a single incident. But Lewis is doing the job, he’s leading the race, he’s creating a big gap in the championship and then he would blow an engine.
“That was tough for him. That was tough for our relationship.”
However, Wolff emphasised that Norris has much longer to recover the deficit than Hamilton did.
“The difference is there’s nine races left,” Wolff said. “Certainly a blow to the championship, but not impossible. That’s tough.”
