Andreas Seidl has quelled expectations McLaren will be front-runners in switching to Mercedes power in 2021.
In the final year of a three-season supply deal with Renault, McLaren clinched third in the constructors' championship, its highest position for eight years.
From next season the team has opted to return to Mercedes with whom it enjoyed a predominantly successful 20-season relationship from 1995 to 2014.
Team principal Seidl, however, has warned it will be impossible for McLaren to haul itself level with Mercedes in a single year.
"Will we get to where Mercedes is? That is not something we will close from one year to next year," he said.
"We still know we have a lot of deficit within the team in terms of the organisation, in terms of infrastructure, which we need to close, first but again, we have a clear plan of how we want to reduce this deficit.
"It will take time but I am confident that, if we do the right things on the team side, that we can close this gap in some years.”
Ahead of the season finale, McLaren announced a £185million cash injection from US-based sports investment group MSP Sports Capital which will enable the team to "play to win", according to McLaren Group executive chairman Paul Walsh.
Seidl has played down the claim, however, adding: “I think we need to stay realistic despite the good result we had in Abu Dhabi and despite the great outcome on the championship delivering P3.
“I think we know exactly where we are. There is still a huge gap to the cars in front, especially to the Mercedes car."
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