McLaren announced on Friday that it would extend its current deal with Mercedes for five more seasons, from 2026 until 2030.
It means McLaren is one of the last teams to commit to an engine manufacturer, with fellow Mercedes partner Williams now the only remaining team to sign up for 2026's rules shift and beyond.
Stella explained that staying with Mercedes was an easy decision because of the continuity it provides and the reassurances it received from Mercedes High Performance Powertrains in Brixworth about how well its 2026 power unit is progressing.
"It was quite straightforward because we are very happy with the ongoing collaboration," Stella said. "They were absolutely instrumental, even in the progress of the team this year, I have to say.
"But above all, the kind of reassurance we got from a technical point of view, the operational standards, just how solid is what we saw, when we checked what was at stake for 2026, made this decision quite simple for us.
"So, we are just delighted that we have this level of continuity and stability as we look forward."
Nailing down an engine supply for 2026 is the next key pillar the team has put in place in its push to get back to the front of the grid.
It follows a technical structure overhaul conducted by Stella, which will be supplanted by the arrival of several high-profile hires this winter, as well as the introduction of a new wind tunnel and other key improvements to its Woking infrastructure.
When asked if the Mercedes powertrain was the final piece of the puzzle for McLaren's 2026 campaign, Stella replied: "I wouldn't say that it's so easy. It's one of the elements you have to have. It's a necessary condition, not sufficient at all.
"In order to contend for championships, there's quite a lot of work to do that needs to be done on the chassis side.
"I think we will be competitive for the years to come from the power unit side, that's why we have made this decision, but there's so much work to do on the chassis.
He added: "Even this year, what we have achieved, yes it's important but already, we haven't developed the car for a few races.
"And we can see that some of the cars are brought some upgrades and immediately our competitiveness is not as good as it was a few races ago. It's an incredibly competitive business."
