Both teams have admitted that McLaren CEO Zak Brown spoke to Horner several weeks ago about the possibility of McLaren using the new Ford-backed Red Bull Powertrains PU in 2026.
Marshall has recently been working at RBP prior to agreeing to join McLaren as technical director, engineering and design, in January.
That led to suggestions that his move could somehow be linked to a PU deal, but Horner insists that RBP won’t be supplying McLaren.
“No, Rob is not going with an engine,” said Horner when asked about the matter by Motorsport.com. “Of course, a couple of teams have approached us about the power unit.
“Our plan at the moment is to supply two teams because we don't want to overstretch ourselves in the first year, so. And of course, those teams are likely to be the two Red Bull-owned teams.”
McLaren team principal Andrea Stella indicated that the Woking outfit is currently likely to stick with Mercedes rather than pursue a deal with Red Bull.
“I can confirm there is no link,” said the Italian. “So we had conversations with Red Bull a few months ago, as part of the due diligence in exploring what's available in the market in terms of a power unit for 2026.
“But at the moment, we are quite advanced in our negotiations with HPP. So there's no conversation ongoing with Red Bull.”
The tone of the announcement of Marshall’s departure from Red Bull suggested that he left on good terms.
Horner confirmed that the 55-year-old is still held in high regard in the Milton Keynes camp – while stressing that he had less of a frontline role in recent seasons.
“Rob has been with us for 17 years,” said Horner. “And he's been an instrumental player in the building of Red Bull Racing.
“And he was able to accommodate things mechanically within the car, like batteries inside the gearbox, with the KERS cars back in the 2009-‘13 era. And he played a significant role within the team.
“But over recent years, he's moved on to other projects, and hasn't been on the mainstream of F1. And after 17 years, he had a significant offer from McLaren. And whilst he still had a period of time left on his contract, he was keen to go back into F1.
“And so we've came to an agreement with him, and I negotiated a deal with Zak that worked for everybody.
“We did a little thing for him at the last debrief. Usually when somebody leaves the team, we tell them to f**k off.
“With Rob, it's a little bit different. He's a good guy, and he's just going on to a new challenge. But it's a little bit like Manchester United, if you look at their team, how it evolved over a period of time, Eric Cantona wasn't still playing 17 years later.”
He added: “He's been working in Red Bull Powertrains of late, but again his work is pretty much complete in the projects he was doing there.
"So it felt right and McLaren could come to a commercial agreement with us that was reasonable enough for us to say we'll let him join in January.”
