With Honda now confirmed at Aston Martin, seven teams either have an internal supply or a works partner, counting both Red Bull and AlphaTauri as recipients of the Ford-backed unit being developed by Red Bull Powertrains.
There are no works deals left for McLaren, Williams or Haas to take.
Aston Martin team principal Mike Krack cautioned recently that a works partner would be crucial in 2026 as the complexity of the engine package means that it will be essential that there is full integration with the chassis side.
Stella says he's not worried about McLaren being a customer as long as it can have a close relationship with a supplier.
The team is currently free to shop around for the best package and is not committed to being a Mercedes customer.
"You definitely have to monitor that in making deals for 2026. That as a chassis manufacturer, like McLaren, you have enough room for influencing the power unit design, so that you are not left behind from a competitiveness point of view," he said when asked about the issue by Motorsport.com.
"So this is what we are doing in our conversations. We know what we want to achieve. To be honest, this doesn't seem to be difficult in the conversations we are having."
Stella stressed that these days customer engine deals have run in the same way as those of works teams, and suggested that any loss of performance due to lack of integration can be made up with a better-performing car.
"You want to ideally be in a position where you can influence all the parameters," he said.
"I think there are two categories. One is the layout: how you integrate the power unit within the car together with the chassis.
"The other one is the performance parameters: how you actually run the power unit. From the second category point of view, we are very comfortable that the FIA already, and I think even more in the future, have in place the conditions to make sure that customer teams and factory teams work with the same parameters.
"When it comes to layout, like I said before, the conversations we are having are definitely taking this point into account. And we are confident that we should be able to get in a strong position.
"If it's a matter of the final milliseconds, we hope we'll be able to compensate for that through chassis development and aerodynamics.
"So we know it's not the absolute best situation, but we don't think is a decisive factor for being successful in the future."
Stella also acknowledged that not having a works deal would give McLaren more freedom to switch if its customer engine wasn't performing well enough.
"This element is definitely part of our conversations," he noted. "So from a contractual point of view, you want to be protected that you will have some criteria of quality criteria of competitiveness.
"And if these criteria are not met then you may even have the possibility to be released from the contract. Like in every good negotiation, this is part of the conversations we are having."
