Carlos Sainz says he will do his best to "lead" McLaren from the cockpit in 2019.
The Spaniard is moving to the struggling British team from Renault.
"I'm leaving very good friends," Sainz said. "I feel that it is unfinished business."
The 24-year-old struggled at times this year, but says the main problem was feeling uncomfortable in the Renault car.
"I did not lose confidence in myself at any time," said Sainz.
Renault boss Cyril Abiteboul agrees that Sainz could return some day, explaining: "The bridges have not been burned, so who knows. He is young and so are we.
"We are very happy that he got another seat and it is at a team like McLaren. It is a team that has to rebuild, but he can take on that responsibility," Abiteboul added.
Sainz agreed: "I'll take my chances with the experience I have to lead the team from the beginning of the year, especially while Lando (Norris) is learning. It will be a new start for McLaren."
McLaren boss Zak Brown says the Woking outfit hit a "low point in the history of McLaren" this year, but is now "on the road to recovery".
McLaren and Renault will use the same engines in 2019.
"We have a very good relationship with them," said Brown. "I got the report about the new power unit in Abu Dhabi and it was very encouraging.
"They've given us a reliable engine and we're happy with the decision," he added, referring to the decision to dump Honda at the end of 2017.
Red Bull is leaving Renault and switching to Honda for 2019, with McLaren's Brown declaring: "We will benefit from that."
