ALLEN PARK -- Few players have been as signature of staples in Matthew Stafford's passing game over the past few years as Theo Riddick. Few have been as durable either.

But the Lions' passing-down running back is battling a knee injury, and his status is starting to appear in doubt for Sunday's road game against the Dolphins.

Riddick missed his second straight day of the practice portion open to the media Thursday. He was on the field before walking off with an assistant, showing a noticeable limp. He missed the entire practice Wednesday, popping up on the injury report with the knee issue that wasn't apparent before the team's bye week.

The Lions back was the only player not practicing at Thursday's open practice portion. Right guard T.J. Lang was out there for a second straight day, a promising sign as he moves through the league's concussion protocol. Ezekiel Ansah also practiced a second straight day. He hasn't played since Week 1 due to a shoulder injury.

Riddick has played in 21 straight games for the Lions despite measuring just 5 feet 8 inches while playing the most physically brutal starting position in football. He's played every game in three of the past four seasons, save for the 2016 campaign where injuries ahead of him increased his rushing workload and ultimately sent him to injured reserve.

The Lions have since found more stable rushing options, such as rookie Kerryon Johnson, limiting Riddick to passing downs. Since he entered the league in 2013, he's been one of the most efficient receiving running backs in the NFL. He's on pace for his fifth straight season with at least 300 receiving yards. He's also the best pass protector the Lions have in the backfield.

If he's unable to go against the Dolphins, that could spell the first offensive workload yet for Ameer Abdullah, who was once the 1B to Riddick in Detroit's backfield. Abdullah could become the preferred option on third downs due to his receiving ability, though LeGarrette Blount has shown to be a more reliable pass protector. Johnson would also get looks for third downs, though the team has been insistent not to overload the rookie so far.

It would present a challenge against a Miami defense that leads the league with 11 interceptions and might get Cameron Wake back to rush off the edge opposite Robert Quinn. Those two combine for 123.5 sacks since 2012, reaching a combined six Pro Bowls in that span, though they haven't reached those production levels yet this season.