With Marshall's signing and McLaren's late-season resurgence which saw them regularly contending for podiums last season, Horner believes McLaren might well be the team to test Red Bull this coming term.
Horner: McLaren could be closest
"I think McLaren had a great second half of the year," Horner told Sky Sports.
"There were times when they were our closest competitor. They have strengthened their team and Rob [Marshall] will be an asset, but it's not just about one person.
"There are seven or eight hundred people in an F1 team and it needs the whole team to come together.
"With Lando [Norris] and Oscar [Piastri] - who was really impressive as a debutant - they could well be a factor next year."
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