Jos Verstappen says he and son Max will talk to Red Bull following their difficult run post-summer break.
The Dutch driver has only one podium in the past four races with the Milton Keynes outfit suddenly dropping back to the third-fastest team behind Ferrari.
Engine penalties at Monza and Sochi largely contributed but even Singapore, a race most thought Verstappen could win, fell flat with a fortuitous third after Mercedes' strategy disaster.
"We should have been closer, but that is more difficult than we thought," Jos said to Ziggo Sport.
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"It is very frustrating. In the last races before the summer break it went really well, but after the summer break, we did not progress while the other teams did.
"Max cannot change that, we depend on the team and the car is a bit behind, the engine is a bit behind, more work has to be done on this combination if we want to challenge for the world title next year."
With Jos previously suggesting Verstappen's future is "open" if Red Bull fall short again in 2020, he sent this warning to team bosses.
“We are very concerned about if there will be any improvement in the short term, we will talk to the team to see what can be done," the former F1 driver continued.
"We depend on the equipment we get and at the moment it is not good enough."
Much of Red Bull's focus has been on fighting for victory at the next race in Suzuka, the home event for engine supplier Honda.
However, Verstappen Sr. believes any victory aspirations are over-hyped.
"Looking at the last couple of races it seems that we will be half a second short in Japan,” he claimed.
"Of course we will try everything to close the gap, but half a second you cannot catch up in two weeks’ time.
"The expectation was to close the gap to the competition, but at the moment it does not look like it. We have to work hard to close the gap, otherwise, next year will be lost too."
