The three-times All-Australian from 2008-10 has suffered a nasty hamstring strain after increasing his running intensity over the past week.
Sandilands, 30, had been widely reported to be nearing full running and on the verge of a return into match conditioning training with a playing return planned for early into April.
He is now expected to be sidelined for up to another month and is will visit a specialist in Melbourne for further assessment of his on-going leg problems.
Sandilands suffered a bad hamstring strain in Fremantle's opening NAB Cup commitments against Geelong in Perth a month ago and was iniitally expected to be out of action for eight weeks.
His latest setback continues a horror injury run for more than two years now.
Sandilands has been plagued with agonising foot injuries over his past two seasons.
It had been understood that the 212cm ruck force was constantly pulling up sore in his feet after heavy running sessions, but his latest setback is a soft tissue complaint in the back of upper leg.
Sandilands was limited to 14 games last season after only 13 in 2011 when he suffered successive foot injuries known as "turf toe" when important ligaments are ripped under the big toe.
Sandilands has played 183 Dockers games and won Fremantle's best and fairest in 2009 as well as runner-up in '08 and '10, when he was virtually unrivalled as the league's premier ruckman.
His career now hangs in the balance with such an on-going succession of injuries for the big ruck force.