McLaren chief operating officer Jonathan Neale says the Woking-based outfit will announce the driver that will partner Carlos Sainz in 2019 by the end of September.
The Spaniard has been chosen to replace countryman Fernando Alonso who will be moving on to hopefully greener pastures next season, but the status of McLaren's second seat is still undetermined.
Earlier this week, Esteban Ocon appeared to be at the top of the team's shortlist, followed by Lando Norris and Stoffel Vandoorne.
The Frenchman and Mercedes protégé visited Woking for preliminary discussions and a seat fitting, but the procedure was apparently cut short because of the inability of McLaren's MCL33 to accommodate the Force India driver's lanky figure.
Reports emerged on Wednesday that Sergio Perez had also entered the McLaren frame, a surprising development given the Mexican's comments last weekend that pointed towards an extension of his deal with Force India.
"We’ve announced that Fernando will be stepping down from Formula 1 and we announced the planned changeover to Carlos Sainz, who will join us from the beginning of 2019," said Neale in a conference call with McLaren investors.
"We’re in discussions about what we will do in the second seat and those announcements will be made in the third quarter."
Ongoing speculation surrounding the drive includes a scenario by which Mercedes may intervene to keep Ocon at Force India to which the German manufacturer supplies its engine.
Lance Stroll's almost certain move to the Silverstone-based squad - perhaps even as soon as the Singapore GP - would leave either Ocon or Perez as the odd man out, hence either driver's potential transfer to McLaren.
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