His biggest, however, came before he even left the locker room.
Jack put on a long-sleeve shirt underneath his No. 44 jersey. That lasted just 30 minutes into the first practice of the Jaguars’ three-day mandatory minicamp when he experienced a steamy Florida summer morning for the first time.
"I was trying to be like the other linebackers and I learned that you’ve got to work your way up to that," Jack said. "I learned as soon as I came out on the field. I took the sleeves off.
"I’m going to be a sleeveless guy until I get acclimated."
That was pretty much the most notable part of Jack’s first full-team workout. Despite the fact that he played at UCLA and worked out over the past five weeks in Arizona, Jack struggled a bit with the heat and humidity. It was more than 90 degrees when practice began in earnest at 10 a.m. ET, and the heat index by the time practice ended was 102.
It wasn’t a dry heat, either.
"This is something different," Jack said with a laugh. "[California] is hot, but this humidity is something else."
Jack had an uneventful day on the field. He received limited reps at middle linebacker as the team tries to ease him into the fold to make up for the five weeks he missed because of the NFL’s graduation rule. He was uncertain at times and looked a bit lost at others, but that was certainly expected because his only interaction with his coaches since May 7 has been via phone.
Learning the defense and his responsibilities on each play via iPad is one thing. Even with the five to eight calls per week the collective bargaining agreement allows, translating that to the practice field is tougher.
"There’s nothing like a real, live rep," linebackers coach Robert Saleh said. "It’s one thing to be in the meeting room and think about your call as we went through it and as we quizzed them, but when you’ve got 10 seconds to get the call in, set the formation, make a check, identify the [offensive] formation that you’re dealing with, get aligned, and the whole process that they go through -- that’s hard.
"That’s hard in a 10-second period, and the only way you can get used to it is through real, live reps."
Jack said the other linebackers have been very helpful in the meeting room and during practice. Jordan Tripp has particularly helped him with the on-field communication. The others are helping with alignment and assignments, too.
"You’re new to it. It’s not a guy that you played with for four years, three years like in college," Telvin Smith said. "You’ve just got to get an understanding. ... Everything just takes time. Playing on the field, making plays takes time and adjusting takes time, so he’ll be fine."
Jack said he’s making up for his lack of experience with one simple rule.
"If all else fails, run," he said. "Just get to the ball. We can fix the other stuff. Your effort is No. 1. So I definitely owe that to those guys, especially my first day, just trying to earn the respect from that group. I owe it to them to just run to the ball and try to be all over the place."
Defensive tackle Roy Miller said he tried to watch Jack on a few plays because he was eager to see what the team’s second-round draft pick looked like. It’s the first time the veteran players have seen Jack on the field.
"He’s an athletic dude, to say the least," Miller said. "I can’t wait to watch the film to see what he did today, but I know today’s his first day. I expect him to make some mistakes. I think a couple days in he’ll be flying around and making the plays we’d thought he’d make."