It follows a period of instability for Horner's team, ultimately culminating in a sharp drop-off in performance last season, as they slipped from being dominant constructors' champions to third place in the standings behind McLaren and Ferrari.
Newey played a huge part in Red Bull's success both with Sebastian Vettel and Max Verstappen, winning six constructors' and eight drivers' titles over 19 years with the Milton Keynes outfit.
Will Red Bull struggle without Newey?
Now, Horner has issued a defiant statement about where the departures leave Red Bull. The Brit's position has come under scrutiny in the last 12 months, following an internal investigation into his conduct which cleared him of any wrongdoing, and calls from Jos Verstappen for him to step down from his role.
However, Horner still believes that the team have what it takes to once again provide four-time champion Verstappen with a championship-winning car, and get themselves back into constructors' championship contention.
"There's only two going and obviously, Adrian left in Miami, so we haven't seen him, he's been working on the RB17 since then, so he's not been working on any F1 projects," Horner told Autosport.
"Obviously, sad to see them go. They’ve both played important roles in the team over their tenure in the team.
"But the show goes on, and I think we’ve got the strength and depth we’ve got. We have that and arguably 2026 - what we're gearing up for in 2026 with our own power unit - is by far the biggest challenge and the most ambitious project this team has ever taken on.
"So, 2025, Jonathan will step off the pitwall, but other than that, everything remains the same.
"I would say rebuild goes way too far for two people that have left. It's evolution. It's something that has been on the cards for a little while, so something that has been part of the planning for some time."