Toto Wolff says Mercedes can fight back in 2019.
Based on winter testing, it appears that the reigning world champions are trailing both Ferrari and Red Bull-Honda.
"We estimate that we are two, three tenths behind Ferrari and two tenths ahead of Mercedes," Dr Helmut Marko, a top Red Bull official, told APA news agency.
Wolff, the Mercedes team boss, insists he is not worried.
"Already last year we were behind Ferrari in terms of pure performance several times and we had to excel to win," he told France's Auto Hebdo.
"It will be necessary to dig deep within ourselves and optimise everything, because we are facing our biggest challenge," Wolff added.
However, Wolff warned that Mercedes must not panic.
"We must be careful not to do too much because that is risky. I have the best team around me. We just have to proceed as we usually do.
"There is a mountain to climb but we know how to do it. We did not become five times world champions for no reason," he said.