But team boss Horner told Italy's La Gazzetta dello Sport: "There are still 15 races left -- it's a marathon not a sprint.
"Hopefully in Spain we will begin to significantly reduce our disadvantage to Mercedes."
He refers to 2012, when Red Bull was also able to recover after a significant rule change.
But the rule change for 2014 has been ever greater, and Mercedes' advantage is obvious.
"Renault is working hard to close the gap," said Horner.
Indeed, Red Bull has made no secret that it believes the only problem with the RB10 is the French-made turbo V6.
"Our car is better than Mercedes, we're just lacking 50 horse power," Roger Benoit, the veteran correspondent for Swiss newspaper Blick, quoted a team insider as saying on Thursday.
Horner confirmed: "I know that our car is fantastic. When we close the disadvantage of the engine we will be back in top form.
"I think that out of the engine suppliers Mercedes has done the best job, followed probably by Ferrari and Renault. But we will see how it is in the end."
Horner was asked if Red Bull could return to the top if it simply changed engine supplier.
"The pressure is obviously on Renault to deliver a competitive product," he answered, "but they have the right people to do that."