CORTLAND — When Joe McKnight shed weight this offseason, he may have gone a bit overboard. He failed his physical earlier in the week and suffered from dehydration in yesterday’s practice, results of having too little body fat, he said.

“I’ve just got to get my body fat up right now,’’ said McKnight, who struggled midway through yesterday’s second practice of Jets training camp at SUNY-Cortland, dropping to one knee during a break and being attended to by a trainer.

The fourth-year running back initially failed his physical administered in Florham Park before the Jets departed for Cortland. He passed later in the week, and was taken off the physically unable to perform list, practicing the past two days.

“I’ve just got to get my body fat up to keep me from cramping a lot, just stay hydrated,” he said. “It’s hard drinking a lot all day because in the middle of the night you have to keep getting up to go to the bathroom, but it’s just something I have to stay focused on and just do it.’’

Two years ago, McKnight added weight in an attempt to become more durable. Then he shed weight to get quicker. He may have shed too much, finishing last season at 4.0 percent body fat and coming into camp at a scant 3.3.

McKnight — who said trainers want him back up to 6.0 — was given a vest containing ice yesterday to help him cool off because he had lost so much fluid.

McKnight also backed up receiver Braylon Edwards’ statements from Thursday that quarterback Mark Sanchez — McKnight’s former teammate as USC — has been inspired to prove his legion of doubters wrong. McKnight said all the criticism in the media and circus surrounding Tim Tebow last season had angered — and inspired — the Jets’ incumbent signal-caller.

Asked if he had seen any difference in Sanchez over the first two days of training camp, McKnight said, “Besides y’all making him mad. But that’s the only thing different I see out of Mark. Mark is always going to be Mark, always going to come out and compete.’’

Asked how the media had made Sanchez mad, McKnight burst into laughter.

“How y’all make Mark mad? Come on, man,” McKnight said with a sideways glance implying the answer was obvious. “All that stuff with Tebow and all that last year. I mean, I don’t know if he was mad, but it kind of put a little pep in his step and want to push for greatness.”