ALLEN PARK -- The Detroit Lions have retooled their defense with some big-time investments this offseason. But they’re going to have to wait a little longer to see what that thing looks like intact.

Snacks Harrison and Darius Slay, two of Detroit’s biggest stars, have not reported for the start of Organized Team Activities this week. Detroit opened OTAs on Monday, and will retake the field for its second session at 11 a.m. Tuesday.

OTAs are voluntary. The only portion of the offseason program that is mandatory is minicamp, which runs June 4-6.

“Those two guys are not here,” coach Matt Patricia said before hitting the fields in Allen Park on Tuesday. “You will not see them at practice today. But we fully expect that this time of the year, that there are guys who are here and guys who are not. And that’s totally fine. We’re working with the guys that are out there. We got a lot of guys out on the field, and we’re working with those guys to get better, and it’s a great opportunity for everyone to improve.”

The reason for the absences is unknown, although a source tells MLive that Harrison is looking for an extension. The Detroit Free Press is now reporting Slay also wants a new deal. Both players have two years left on their current contracts.

Harrison transformed the Lions defense after arriving in an October trade last year. He racked up 30 run stops, which led the team even though he played in just 10 games for Detroit. He was the best interior run defender in the league, according to ProFootballFocus, and his influence is undeniable.

Detroit ranked 30th against the run before the October trade, then rocketed up to 10th by season’s end. It allowed just 925 rushing yards after the trade, fourth best in the league, and 3.76 yards per carry. That was second.

But Harrison is also 30 years old. He’ll turn 31 in the middle of next season, when he makes a base salary of $6.75 million. He’s scheduled to make another another $9 million in 2020, and has $250,000 workout bonuses for both seasons.

With so much money already invested in a player who will be 32 when his current deal expires, Detroit may not feel an urgency to get something done this offseason. General manager Bob Quinn punted on the opportunity to weigh in on Harrison’s situation in March at the owners meetings.

"My priority right now is the draft,” Quinn said then. “We got another month or so before the draft is here, so that (extension) is something that we’ll probably talk about in May or June.”

As for Slay, he’s two years into the four-year, $48.15 million extension he signed before the 2016 season. And he’s been worth every penny of it, making the Pro Bowl in each of the last two years and earning All-Pro recognition in 2017.

He’s due to make $12.55 million in base salary this year, plus has a roster bonus of $234,375 and workout bonus of $250,000. He’s scheduled to make another $10 million in 2020, with a roster bonus of $250,000 and a workout bonus of $250,000.