Red Bull's Helmut Marko has expressed concern about Max Verstappen's future with the Formula One team if they fail to give the four times world champion a faster car.
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Verstappen, winner in Japan on April 6, struggled to sixth place in Sunday's Bahrain Grand Prix and at one point was last on track.
The year before the 27-year-old Dutch driver had won from pole position at Sakhir, for the second year in succession, with the fastest lap thrown in.
He is now third overall and eight points adrift of McLaren's leader Lando Norris.
"The concern is great. Improvements have to come in the near future so that he has a car with which he can win again," motorsport consultant Marko told Sky Germany, who reported the fear of losing Verstappen to another team even for next season was growing.
"We have to create a basis with a car so that he can fight for the world championship."
Britain's Sky Sports television said Verstappen's manager Raymond Vermeulen was seen shouting at Marko in the team garage after the race.
Dutch reporters quoted Verstappen as telling them he felt he was now just competing in the championship rather than fighting for it.
The reigning champion has a contract with Red Bull to the end of 2028 but Marko has confirmed previously that the driver has a release clause.
He has been linked to both Mercedes and Aston Martin, the latter now the team of former Red Bull technical head Adrian Newey and future partners of Red Bull's current engine supplier Honda.
After starting seventh, Verstappen's race was hit further by a problem with the team's overhead light system that signals when a driver can leave the pit box.
"It was a bad weekend for the team, nothing went our way from the start of the race," team boss Christian Horner told reporters.
"But it's a 24 race championship, we're eight points behind in the drivers championship and we know we need to make progress very quickly."
The next race is the Saudi Arabian Grand Prix in Jeddah this weekend.