Now we know why Tahir Whitehead missed OTAs.

The Detroit Lions start mandatory minicamp on Tuesday, but it’s unclear whether they’ll be with or without the leading tackler from 2016. Linebacker Tahir Whitehead Missed all organized team activities from the past three weeks while getting excused absences from the coaching staff.

The reason for Whitehead’s absence was a mystery, but Detroit Free Press’ Dave Birkett shed some light on the situation on Monday:

Tahir Whitehead hasn’t taken part in any of the three OTAs open to reporters as he’s been working through a lingering knee Injury. Whitehead wasn’t even at last week’s voluntary workout

This is the first we’ve heard of a knee injury for Whitehead. In 2016, Whitehead briefly appeared on the injury report with a knee injury that caused him to miss Week 13’s matchup against the Saints. He was on the injury report for one more week with the same injury, but was off the list completely for the final four games of the season.

Though he missed all of OTAs, the injury doesn’t appear to be too serious. Last week defensive coordinator Teryl Austin said he doesn’t expect Whitehead to be out much longer. “He’s been here. He’s doing some things in the offseason, but he’s fine,” Austin said. “I expect him to be ready to go when camp starts.”

Whenever Whitehead does return to practice, he will be moving back outside to the WILL linebacker position. The Lions drafted Jarrad Davis in the first round this year and Davis has been getting all of his reps at middle linebacker during OTAs. Austin confirmed the switch for Whitehead last week. “Probably will be playing the WILL linebacker and he’s fine with it,” Austin said.