Marquez, who is well-placed to take a sixth premier class title this weekend, had a heavy high-side crash at Turn 7 in FP1.
Shaken up and limping after the crash, the Honda rider was transferred to a local hospital but was cleared to return for FP2, where he set what was then the best time of the weekend in three laps and before eventually taking sixth.
Marquez said that although it is "painful everywhere" for him, he quickly recovered after the crash and the main worry was not being able to breathe for five seconds.
"Was around five seconds that I couldn't breath," recalled Marquez. "For that reason I was there on the floor, on the gravel. It is only five seconds but for me was like 20 seconds.
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"Was a big impact and it is true that step by step, [I got] better and better and when I arrive in the medical centre, I was already okay.
"But then, I fully understand, the doctors try to manage the situation in the best way and they prepare to have a deep scan about all these things and we went in the hospital but the most important thing is that I am okay.
"Painful everywhere, but the biggest thing after the crash was that I couldn't breathe in a good way. This was the main problem."
Marquez explained that when he crashed, he completely closed the throttle at Turn 7, which is something he normally doesn't do at the tricky right-hander.
"I was much slower than the normal laps because I was [coming] out of the box," he explained. "Was new tyres, maybe the track was not clean enough but it is normal.
"The main difference is that normally on that corner, we don't close completely the gas. On that lap I close completely because I went out of the box and I was not pushing.
"The outlap, I close the gas. When you close the gas, engine brake is going in, and then is when I lock the rear.
"It's not a mechanical problem, is more because was my mistake, but because I was riding slow.
"A bit more than this change of the riding, was more maybe the tyre was not ready or maybe was a little bit on the dirty place. Difficult to understand."
Jack Miller said that it is "very easy" to have a crash like Marquez especially at Turn 7, which the Australian takes like if it was wet.
"Very easy to do that," the Pramac Ducati rider said. "Honestly, on a new tyre you go out there and because so many hard right-hand corners here, the right-hand side of the tyre you really gotta be careful on that first lap. It caught him out there.
"I'm going through there [Turn 7] like it is in the wet, keeping a tiny bit of partial throttle not to upset the bike at all.
"It is slippy, it is not a nice point, you saw [Miguel] Oliveira also go down there this morning on the outlap."
Additional reporting by Oriol Puigdemont and Valentin Khorounzhiy