Perth Glory defender Michael Thwaite is facing a sizeable stint on the sidelines after injuring his calf in Thursday night's 1-1 A-League draw with Western Sydney at Patersons Stadium.Thwaite felt a pop in his right calf while preparing to jump for a header in the 77th minute.The 29-year-old was on crutches after the game, and will undergo scans on Friday to determine the extent of the damage.Thwaite has played a key role for the Glory in defence this season, with his excellent form resulting in a Socceroos call-up for the recent East Asian Cup qualifying campaign.Glory coach Ian Ferguson said Thwaite's absence would open the door for Steve Pantelidis to return to the starting line-up."He felt a pop so it doesn't look too good at this moment in time," Ferguson said of Thwaite's injury."He's been a rock up the back for us, him and Bas (van den Brink)."He's been great all season. Missing his experience is obviously going to be a blow."But Panta's a big-time player. He's come in before and done a great job for us. I have no qualms throwing him in there at all."Glory winger Travis Dodd (knee) is only a 50-50 chance to return for Monday night's clash with ladder-leading Central Coast at Bluetongue Stadium, but left back Scott Jamieson is available after serving his one-match ban.The Mariners will be without three key players against the Glory after John Hutchinson and Tomas Rogic were handed red cards in Thursday night's 1-0 loss to Sydney, and striker Daniel McBreen picked up his fifth yellow card of the season.