The 35-year-old had signed a two-year contract extension in January and was one of the team's best performers last season,although his regular heroics in the goalmouth were not enough to avoid a disappointing ninth-placed finish.
Lopar did not participate in the season restart,but flew back to Sydney in October and endured two weeks in hotel quarantine to begin pre-season training.
Robinson said he would be"silly"if he wasn't searching for another custodian,especially with back-up Vedran Janjetovic still struggling with a long-term shoulder injury and unlikely to resume training anytime soon.
But whether he signs a number one or number two will probably hinge on how well Margush,23,handles the opening rounds of the new season,which will be officially launched on Monday with a Fox Sports-controlled event at Darling Harbour.
Margush has featured only seven times at A-League level in previous stints at Adelaide United and Perth Glory,but Robinson believes he has serious potential.
"I'm very,very confident in Daniel Margush. I liked him when I first came over,his name got mentioned to me when I was at my previous club Newcastle but I wasn't looking for a young goalkeeper,"Robinson said.