He is likely to return after the club's mid-season bye in Round 11.
Cassisi injured his hamstring in the third minute of last Sunday's three-goal loss to Carlton at Etihad Stadium.
Injury Update: Monday 20 May
High performance manager Darren Burgess told portadelaidefc.com.au Cassisi will likely miss through to the club's away match against the GWS Giants.
"It's probably a three to four week injury, but he's had hamstring problems before," Burgess said.
"We're conscious of that, and while he could recover in three weeks, it's more likely to be four considering his history."
"That would mean the GWS game."
portadelaidefc.com.au gives you the inside word on the Power's injury list every Monday night in-season.
