He never mentioned it to reporters, word never leaked out of the locker room. Even now, he's hesitant to blame the illness for his subpar 2014.
But coach Bruce Arians has no doubt it affected his play.
"He was heavier than he'd been and it was hard to control his weight," Arians said. "So I think that had a lot to do with it, early on."
A noticeably slimmed-down Peterson said the illness is under control.
"I still have to watch what I eat, my sugar level, take my blood pressure and things like that," he said.
Although he said he doesn't want to use diabetes as an excuse for what he termed a "disappointing" season, Peterson acknowledged that it affected his play.
"I was just a step behind from making those plays, and that was due to my weight," he said. "That was due to sluggish movements and things like that. Now all of that is under control. That's all behind me and it won't happen again."
Peterson, listed at 219 pounds, won't say how much weight he lost.
"I lost a lot," he said. "I lost enough."
Before last season, Peterson signed a five-year, $70 million contract, with $48 million guaranteed. Failing to show he was worth it last year, diabetes or not, has him highly motivated.
"I think that everything he went through last year, getting a big payday and not really playing well, he definitely took this offseason to get healthy and get into the film room, and it's really showing in practice," teammate and friend Tyrann Mathieu said.
Mathieu said he thought the illness "played a big part" in Peterson's difficulties.
"Just his confidence and things like that," Mathieu said.
Not that the season was a total disaster.
Peterson held Dez Bryant to two catches for 15 yards in Arizona's 28-17 win at Dallas last Nov. 2. Playing without Tony Romo, backup Brandon Weeden targeted Bryant on 10 passes but didn't complete one to him until less than two minutes remained in the game. The following week against St. Louis, Peterson intercepted two passes, returning one 30 yards for a touchdown. But he had only one other interception all season.
Still, he made the Pro Bowl for the fourth time in his four NFL seasons. He's the fourth player in NFL history to appear in four Pro Bowls before the age of 25. The others: Jim Brown, Barry Sanders and Emmitt Smith.
But, as Peterson said, "Enough about last year. "
This year, he said, "I feel great."
"My technique feels unbelievable," he said.
Peterson and Mathieu, who's coming off a season slowed by injuries, have been energized and dominant players so far in camp.
"Me and Pat had a talk after the season,'" Mathieu said. "You know, for me and him, we're two playmakers in our secondary. We have to perform at a high level and kind of raise the level of play for our entire defense. We really want to put that pressure on our shoulders and really perform how we're supposed to."
Peterson called the secondary "the heart and soul of the defense."
"Everything runs through us," he said. "The linebackers, the d-line can make a mistake all day. No one will ever see it. But as soon as we make a mistake, someone gets behind us, the band's playing, it's a top 10 play on ESPN."
NOTES: The Cardinals had a scare on Monday when, on their first day in pads, Mathieu pushed running back Kerwynn Williams into Carson Palmer, knocking the quarterback to the turf. Fortunately, Palmer quickly got back up. For an instant, Mathieu said, "My heart was in my behind."... Palmer didn't practice Tuesday. Arians said it was a scheduled day off and that backup Drew Stanton will have Wednesday off. ... The Cardinals released cornerback Damond Smith and signed cornerback Jonte Green. Green has appeared in 24 NFL games. He was released by Buffalo on April 25. ... First-round draft pick OT D.J. Humphries and OG Earl Watford limped off the practice field on Tuesday. Details of their injuries won't be known until Wednesday. ... Arians ended Tuesday's practice 25 minutes early. OLB Alex Okafor said the coach promised he'd cut them loose if they "finished the script" early.