MYSTERY surrounds the severity of a knee injury to premiership Tiger Shaun Grigg after he failed to train at Punt Road on Thursday. 

Described as "medium term" by Richmond, the 31-year-old has been reduced to light duties in recent weeks and only completing straight-line running away from the main group.

By comparison, Josh Caddy (ankle) and Tom Lynch (knee) remain 1-2 weeks away from full match simulation and have been listed as "short term" by the Tigers' medical staff.

It places Grigg's hopes for the season-opener against Carlton in 28 days in doubt.

Grigg has been incredibly durable for the Tigers since crossing from Carlton at the end of 2010, playing 171 of a possible 184 games with a hamstring injury in 2014 his only major setback.

FULL INJURY LIST Who's racing the clock for round one?

He provided the back-up option to Toby Nankervis during the Tigers' breakthrough flag in 2017 before again carrying the load as the No.2 throughout last season.

Whether he plays in round one or not, the Tigers are expected to rethink their ruck set-up under new AFL rules introduced for 2019. 

Caddy completed little on Thursday as Lynch stepped up his rehab from knee surgery which has him in a race against time for the clash with the Blues. 

Jason Castagna continues to build his running base after an ankle setback, while Jacob Townsend (thigh) and Ryan Garthwaite(Achilles) were on modified programs.

Jack Riewoldt, Jack Higgins and Alex Rance were no-shows as they prepare for the realestate.com.au AFLX tournament on Friday night.

The Tigers will take part in an intraclub on Saturday at Punt Road.