Eric Boullier says the new front wing McLaren has introduced for this weekend's United States Grand Prix is part of package focused on developing its 2018 Formula 1 car.
Fernando Alonso - who will start the race at Austin in eighth - has the new wing, which is part of a series of recent additions that includes a new floor and bargeboards.
"We switched early to next year's car," said McLaren racing director Boullier.
"We're working in a R&D mode. We keep developing for next year's car.
"We bring some parts for it. [The front wing] is part of this package. It looks like it works well with the full package, even if we need to now go to the next step."
When asked if Stoffel Vandoorne, who will start 15th after a five-place grid penalty for an engine component change, will get the new front wing in Mexico, Boullier said: "We are trying our best.
"We have a different model [for the] way we want to work this year. He may have in Mexico, he may not.
"It will be confirmed in the next 48 hours. If not, he'll definitely have it for Brazil."
Alonso was pleased with the performance of the new wing and believes it bodes well for next season, as McLaren targets a big step up the grid when it switches from Honda to Renault power.
"I think it was great," said the double world champion. "It was definitely a step forward.
"Every single race we bring some updates and we brought some things for the front of the car and they seem to work as we expected, so this is probably the best news for next year.
"Everything that we put in the windtunnel we put on the car and it immediately works so definitely with the development we are doing for next year we should be in the right direction."
Vandoorne said there was "not a huge difference" in performance of the new updated spec 3.8 Honda engine.
Honda F1 project leader Yusuke Hasegawa added: "There's a difference, from a power point of view but it's fair to say it's not as big so the driver can feel."
Hasegawa said it is uncertain if Alonso will get the new engine as his current unit can get to the end of the year and the Japanese manufacturer is keen to avoid penalties.
"Fernando's engine has enough mileage to get to the end of the season so we have no plan to introduce it," he said.
"But if we have a chance, we will introduce it. We're not stopping any development."