"Now they want to 'kill' Sebastian because he's not right for the Red Bull image," said the French Canadian, amid rumours McLaren has made an early approach to Vettel.
But Red Bull boss Horner has tipped the team's reigning four-time world champion to bounce back, essentially blaming the bulk of the 26-year-old's struggles on "bad luck".
"Sebastian has had an incredible amount of bad luck this year," he told the Austrian broadcaster Servus TV. "Three times he's failed to finish but he's had many more technical problems than that."
Despite Vettel's struggle simply for a functioning car, Horner says the German is dealing with the situation "incredibly well".
"Many other drivers would have their heads down, their motivation would be gone. Not him. He works harder and pushes the team even more," he added.
"When the tide turns, the results will come back," said Horner.
According to F1 legend Niki Lauda, however, a major element to the Red Bull situation in 2014 has been Daniel Ricciardo, who surprised the entire paddock with his rapid rise among the very best drivers in formula one today.
"He is a massive talent that has reached a very high level in a very short period of time," said the triple world champion, who is Red Bull rival Mercedes' team chairman.
"He has everything it takes to be a world champion in the future," added Lauda.