Image:-McLaren's boss expect Sebastian Vettel to leave F1
He expected Vettel to retire from Ferrari by the end of 2020.
"Obviously Seb is a phenomenal driver and a four-time champion," the American told Sky Sports TV.
"But in the off-season, I think we were pretty far down the path and knew either Daniel or Carlos would land with us.
"We never really entertained anyone beyond that, and especially with the late breaking news from Seb - at that point we were pretty far down the path."
Ferrari said Tuesday that Vettel, who won all of his Red Bull titles between 2010-13, will leave with them after six years.
Two days later, McLaren announced that Australian Ricciardo would join them from Renault in 2021 with Sainz heading to Maranello on a two-year deal with partner Charles Leclerc.
In 2018, when the seven-time race winner was at Red Bull mulling his options, former world champions McLaren tried to sign Ricciardo, and Brown said they had "stayed close."
Brown added that if Vettel wanted to continue it had limited options.
"It doesn't look like there's a chance for him on Mercedes or Red Bull, there's no McLaren opportunity for him, obviously not at Ferrari, so the next best is Renault," he said.
"And the next question is whether Seb wants to go with a team that won't likely win in 2021?
"It's a great team and they should move up the grid too, but I think if Seb doesn't want to restart a journey with a team back on a journey - then I think he will probably end up leaving the sport unfortunately."
Last year, Renault finished fifth overall behind McLaren, who switched from the engines of the French manufacturer to Mercedes in 2021.
Mercedes, Ferrari and Red Bull are the only teams to have won races since the start of their 1.6-liter V6 turbo hybrid era in 2014.
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