Limited to individual work throughout the offseason program, Jaguars linebacker Myles Jack compared observing team drills from the sideline to "sitting in the house and watching all your friends go play."

One thing that does put a smile on Jack's face: He still has a lot of friends on the Jaguars' defense.

Jack noticed that during a walk-through to start Tuesday's practice, the first time since last season ended that the entire starting unit was on the field together.

Of the 12 players who saw the most defensive snaps for the Jaguars in 2017, 10 are back. Only cornerback Aaron Colvin (signed with Houston) and linebacker Paul Posluszny (retired) are gone.

"Just seeing everybody in there, all the faces that you're used to seeing when you're calling the play, it was cool having everybody back in," Jack said. "You can definitely feel that."

The Jaguars took a cautious approach with Jack during 10 organized team activities and the three-day minicamp that ended their offseason program Thursday. Jack participated in walk-throughs to install the defense and worked individually and on the side with a position coach, but 11-on-11 work and other team sessions were out.

Jaguars coach Doug Marrone said his rationale for that choice was "not a medical decision." Marrone expects Jack to be a full participant when training camp opens July 26 but wanted to ease him through the offseason.

"He hasn't had a lot of time to condition as much as I would like him to do to put him in there," Marrone said. "I think he's ready to go. I just didn't want to take a chance of putting him in there and not putting him at the best he can be from a conditioning standpoint."

Jack hasn't been on the field for team drills, but he has been learning.

This is an important offseason for the 22-year-old, who is expected to replace Posluszny and become the full-time middle linebacker. Last season, Jack played the strong-side spot on base downs and moved to the middle when the Jaguars had five defensive backs on the field.

That worked out well. Jack finished second on the team with 90 tackles and added three passes defensed, two sacks and a fumble recovery. But Posluszny's decision to retire earlier this year created a gap at middle linebacker, and Jack has been studying to fill it.

"You're always in the mixture here," said Jack, who is entering his third season. "Before, when I was playing [strong-side linebacker] in base, if the ball went to the other side of the field, there was no way to make a play. Now being [middle linebacker], you're always around the ball. You're always in the mixture. Run and pass, you're pretty much always around the ball. It's going to be fun. But it's going to be a lot of work."

Said defensive coordinator Todd Wash: "He is fantastic in the pass game. But the adjustments of playing inside in the run game, we will get all of that when we get into training camp."

The Jaguars will have two weeks of practice from the start of training camp until their preseason opener against New Orleans on Aug. 9. And there will be plenty to get done.

In addition to getting Jack up to speed at middle linebacker, the Jaguars will need to decide on a starting strong-side linebacker. That's the player who will typically come off the field when the Jaguars are in their nickel defense, but it's a crucial spot on early downs.

Second-year linebacker Blair Brown seemed to be the early candidate to win the job, but rookie Leon Jacobs performed well during the offseason program and could be a consideration.

"We feel that we have six or seven guys [at linebacker] that can really play," Wash said. "We will let that shake out once we get into training camp.”

Whoever ends up winning the job next to Jack and weak-side linebacker Telvin Smith will be joining a defense that carries big expectations into the season.

The Jaguars finished second to Minnesota in scoring and yards allowed last season and feel they are ahead of schedule because so much of the defense returned.

From the middle of that group, Jack will be a critical part.

"You're only as good as your last work, so we need to be better," Jack said. "That's the standard and that's the expectation. We all expect to go out and make plays and get three-and-outs and change the game. That's the mentality of the guys on defense. Anybody can change the game. At all 11 spots. We've got a lot of talent."

Phillip Heilman: (904) 359-4063