The potential McLaren IndyCar programme would be set-up by affiliation with an existing outfit if the Formula 1 team commits to the series full-time in 2019, Autosport understands.
McLaren set itself a September deadline to decide if it will race in IndyCar, compete in the Indianapolis 500 once again or purely focus on F1.
Two-time F1 world champion Fernando Alonso completed his first test of a 2018 Indycar at Barber Motorsports Park on Wednesday, running in an Andretti set-up.
McLaren's most likely option for a full season would be to reunite with Andretti after the partnership put Alonso in contention to win the Indy 500 at his first attempt.
A takeover of the financially struggling Harding Racing has been mooted, but the team has pushed ahead with its 2019 plans and is set to run Indy Lights title-winner Patricio O'Ward and his rival Colton Herta at Sonoma next weekend.
McLaren told Autosport that "nothing has been decided" and the situation has been described as "fluid" by various sources.
Autosport understands IndyCar chassis provider Dallara has had no approach from McLaren regarding a chassis supply, and that the series is continuing to support McLaren's ambitions.
An affiliation would allow McLaren to shortcut the learning process and lessen the need for an instant recruitment drive, allowing it to use the data and equipment of a pre-existing IndyCar operation.
Penske team president Tim Cindric told Autosport he would "love" McLaren to join IndyCar as its own entity, saying: "Indycars are so similar and the areas of opportunity are so small that I'm surprised that McLaren see that it's important to really have any affiliation or partnership, just from the outside.
"Maybe there's more to it. But the McLaren brand, their name, and the fact that they have a free car that's also branded in the same way seems as though it muddies the water when you try to tie that to another organisation.
"I would be surprised if they take that approach but, if they do, I can only assume it's to try to have access to experienced people in IndyCar itself.
"I think, just like with anything else, people make the difference. In IndyCar, it does more so than maybe Formula 1, in the sense that the numbers are quite a bit different because of the areas of open development are so constrained relative to Formula 1.
"I think that the qualified people, they [McLaren] would be more than able to attract the right kind of people in a short period of time."
Affiliations have become more common in IndyCar, with the likes of Vasser-Sullivan's partnership with Dale Coyne Racing's Sebastien Bourdais and Michael Shank Racing's Schmidt Peterson Motorsports-affiliated entry with Jack Harvey proving successful.