The midfielder had an MRI scan yesterday that revealed damage to his anterior cruciate ligament; the same one he damaged in 2006.
Pantelis hurt his knee in the 3-0 loss to the Central Coast Mariners on Saturday night.
Reds coach Aurelio Vidmar said the 26-year old would be a massive loss.
“Unfortunately Lucas will be out for at least 4-6 weeks and it is devastating for him and the club, because he worked exceptionally hard to get back from the knee injury he had a couple of years ago,” Vidmar said.
“Hopefully we’ll see him back on the park in the near future.”