Fernando Alonso left Bahrain expecting McLaren-Honda to make a "big step" for the start of the forthcoming European season.
Having switched from the now-resurgent Ferrari, the Spaniard - as well as fellow champion teammate Jenson Button - have struggled so far in 2015 and are the only drivers except those at Manor yet to score a single point.
But a car and engine upgrade are now due for Barcelona, Alonso's home race and the first event after the opening quartet of 'flyaway' races.
El Confidencial newspaper quoted team boss Eric Boullier as saying "most of the back of the car" will be "new" in Spain.
"In Spain we will see the first big step forward," Alonso said.
But Boullier warned: "Any step would be good.
"We cannot expect to take a big step," he added. "Those days are gone. It's more a case of gradual improvements in this modern formula one."
Former long-time McLaren driver Mika Hakkinen, who won both of his world championships wearing grey, wonders if Alonso has the patience to wait for the team's next phase of success.
"You are right," he told a reporter for the Daily Express, "is Fernando ready to work next two, three, maybe four years in a situation when you are not close to winning?
"It can be a really boring process because it's not only 15, 16 races in the season, it's a long year, travelling around the world, it's really, really tough, so can he motivate himself all the time?" Hakkinen added.