The former Power captain was substituted from Saturday night's 35-point win over Collingwood early in the first term.
Senior coach Ken Hinkley confirmed Cassisi's injury after the game.
"It's disappointing, he's got a hamstring injury and Dom's been significant for us even though he's again got hurt today."
"I do know Dom will go and rehab really hard, really well and get himself ready to come back in.
"He's had interruptions from just before Christmas and paid a bit of a price (for that)."
Tom Logan suffered a corked calf during the match and Travis Boak left the field with an ankle complaint, but both are expected to be fit for Sunday's away game against Essendon at Etihad Stadium.
portadelaidefc.com.au will have a complete report on the Power's injury situation on Monday.
