Jos Verstappen says he and his son are starting to doubt Red Bull Racing's capability to deliver a winning car powered by Renault.
Max Verstappen suffered his sixth retirement of the season at the Belgian GP on Sunday, his fourth DNF caused by a mechanical failure.
While both father and son remain committed to the Milton Keynes-based outfit, frustrations and questions are inevitably mounting.
"Maybe this is the wrong moment to ask this question, but you do start to question everything," Jos Verstappen told Dutch TV channel Ziggo Sport when asked if a future without Red Bull was now being considered.
"I notice about Max that he's very disappointed. It's tough to keep yourself motivated the whole time when things are going like this.
"I mean, he's doing very well in qualifying, he's half a second quicker than his team mate and he's just behind the Ferraris.
"And then, after seven or eight laps, he's standing at the side of the track again. This should not be able to happen, certainly not at a top team."
Verstappen also dispelled the notion that his son's reliability woes were in any way rooted in his driving style.
"Max isn't doing anything wrong," added Jos "People are asking that question again if Max is being too tough on his equipment. But that's complete nonsense.
"He can't overdrive, he can't break the material. We all know what it is: it's just the equipment that isn't good enough.
"I think we all need to calm down for a bit and then maybe we need to sit down with each other because this won't last if things keep going like this."