The Red Bull KTM rider was running seventh on Lap 3 when he went down at Turn 10 of the Circuit de Barcelona-Catalunya.
Miller has now failed to score four times in six Grand Prix races so far in 2024, having crashed out for the last two in a row, been taken out at Jerez, and remounted to finish only 21st at Lusail.
“Obviously, extremely disappointed. Let the team down and let myself down,” he said.
“We are trying to be better and stop these silly mistakes.
“I just trusted the front too much, too early and I tried to carry the corner speed as I had done all weekend but the track conditions were different this weekend.
“Without warning it disappeared. There was not much I could do. A tough one.
“It had been nice, running with the boys again, we just need to get that confidence in the front up again. We’ll keep working.
“It’s a testing time in my career but with the right people around me I know I can right the ship and come back stronger in the races to come.”
Miller, who is coming off-contract, had already been under pressure when KTM Motorsports Director Pit Beirer said his performance has been “a little bit lacking … for a MotoGP seat.”
He had little choice but to acknowledge the threat to his seat on media day at Catalunya, but has now slipped even further to 15th in the riders’ championship.
“A bit of a shame for Jack with an early crash and he didn’t make any strange move,” said Red Bull KTM Factory Racing Team Manager, Francesco Guidotti.
“We know the grip is very low here and maybe at the start of the race he didn’t have the pace he wanted and tried to catch the group.
“No big drama because it had been a good weekend otherwise.
“Mugello is one of his favorite tracks so let’s see if he can show his real potential there.”
Brad Binder led the way for the KTM/GasGas quartet on Sunday in Barcelona with a finish of eighth, while GasGas Tech3 rookie Pedro Acosta remounted after crashing from second place to be classified 13th in the end.
The Italian Grand Prix takes place at Mugello next weekend (May 31-June 2).