IOWA CITY, Iowa (AP) — The biggest hole Iowa will have to fill by September is at linebacker, where three multi-year starters have graduated and are preparing for their shot in the NFL.

The Hawkeyes appear to have gotten some clarity in the middle of their defense this spring.

Defensive coordinator Phil Parker said Tuesday that Nick Niemann, Amani Jones and Aaron Mends have put themselves in position for starting jobs as spring workouts wrap up this week with an open practice on Friday at Kinnick Stadium.

Though the Hawkeyes also seem comfortable with Jack Hockaday, Kristian Welch and sophomore Barrington Wade, it remains unclear if Iowa will stick largely with three starters like they did a year ago or use a larger rotation. Iowa opens 2018 by hosting Northern Illinois on Sept. 1.

"I think it's too early to tell," Parker said. "I like to settle in on — there's going to be somebody that's going to be the starter. So whoever that is, I think you play them until you think the other guy is better in, my opinion."

The biggest piece Iowa must replace is middle linebacker and All-American Josey Jewell. But the 6-foot, 240-pound Jones appears to have taken a step forward this spring, with Parker praising both his commitment and his enthusiasm in a press conference on Tuesday.

"I think his ability to read the box and diagnose the plays (is impressive), and when he goes, he goes," Parker said. "I think his ability to go ahead and trigger when he sees something, and when he comes, he's well-packed. He's got a low center of gravity, and he runs through guys."

It should help Iowa's new linebackers that its defensive line has proven to be a major strength this spring. The Hawkeyes could have one of their best units in years with three starters back and plenty of depth.

Senior end Parker Hesse will miss Friday's workout with an undisclosed injury that Parker said isn't serious. That could mean more snaps for sophomore A.J. Epenesa, perhaps the most highly-touted recruit coach Kirk Ferentz has ever landed.

Epenesa had 4.5 sacks and earned all-freshman honors from the Big Ten in 2017 despite not joining the program until last summer. If the former five-star recruit can live up to his promise in 2018, the Hawkeyes' line might be as good as any in the Big Ten and beyond.

The 6-foot-5, 277-pound Epenesa has shown the ability to play both right and left end this spring. Still, his youth has also showed up at times during workouts.

Iowa will also return all-league candidates Anthony Nelson, a junior, and Matt Nelson, a senior.

"A.J. is a very talented guy. There are times where I think he's really done a good job," Parker said. "He's still young and understanding the position ... (but) he still has a long ways to go to where I think he needs to be. He's got to be a little more disciplined in knowing his responsibility and understand that people are going to change formations, they're going to change the calls for you."