Cassisi injured his knee in the second quarter and the tough midfielder had his right knee strapped in an attempt to play on; but he emerged from the rooms at the main change wearing the red substitute vest.
He will have scans on Sunday, but Primus said it was possible the inspirational leader would require surgery to repair the damage to his knee.
"Dom will see a specialist and have a scan tomorrow, but he's probably going to be out for a few weeks. He might need an op on his knee," Primus said after the game.
"It's a medial or something to do with the meniscus in his knee. We're not quite sure. We'll find out tomorrow once he has the scan."
Defenders Paul Stewart and Jasper Pittard also battled knee complaints, but the club says both injuries are only minor.
The Power started slowly on Saturday, with only a brilliant individual effort from Robbie Gray and West Coast's inaccuracy in front of goal keeping them in the game.
Stars Travis Boak, David Rodan and Danyle Pearce struggled to get near the ball in the first half, while Eagles midfielders Andrew Embley and Luke Shuey were allowed to dominate.
Primus said his onballers simply didn't work hard enough to win the ball, particularly in the opening half of the game.
"We got blown away early…and we watched the ball a little bit. It was really disappointing and probably even more disappointing was to fight back [because the players showed] they actually had it in them to win the ball," he said.
"West Coast just outworked us. We were struggling for periods there with Dom coming off and Paul Stewart was out of action for a while too. We just couldn't keep our work rate up as much as they could and hence it came back to bite us on the backside in the second half.
"Our contested stuff is pretty good. We just don't have enough of the uncontested stuff. We're playing a pretty intense game at the moment and we don't have enough work rate of players to find enough easy ball."
Primus will have a few options at the selection table this week, with forwards Daniel Motlop (five goals) and Brett Ebert and defender Jacob Surjan all pushing for a recall against Geelong.
