Not if it was just statistics that said he was bad. He would never be benched for bad preseason game day performance.
But if the performance in the preseason you revealed some underlying problem, such a severe reduction in arm strength or loss of Mobility, and it is clear that Tom Brady had significantly lost Talent, I think there is a level of decline that might be revealed in the preseason that would cause Belichick to start Garoppolo.
Now I'm talking about something pretty severe here a complete loss of ability to repeat his mechanics combined with a severe loss of arm strength and obviously struggling to regain form. It would take something absolutely meteoric to dislodge Tom Brady at this point. But to say it's absolutely impossible is probably wrong.