IRVINE, Calif. — For a third consecutive day here at Great Park in Southern California, U.S. soccer star Christian Pulisic trained separately from his World Cup teammates.He was spotted in a makeshift gym as Wednesday’s session began, this time with some sort of compression sleeve around his injured left calf area.Eventually, after a series of body-weight exercises and moderate-speed movements, he jogged out to a field for some light passing with a member of the U.S. team’s performance staff. A team spokesman said that Pulisic remains “day to day.” Teammates have said he’ll be fine, but as mystery — and perhaps gamesmanship — swirls around Pulisic’s status, the possibility that he might miss Friday’s World Cup match against Australia has at least become real.And so, the conversation around Pulisic’s health has drifted toward an additional question: If he can’t play, who would replace him?Pulisic, in some sense, is irreplaceable. He is the USMNT’s attacking catalyst, its best player and one of its three most important. On paper, he is integral to any U.S. game plan for Friday’s match against Australia, which could ultimately decide the Group D winner.But on the other hand, the World Cup is a long tournament. And the last time the U.S. met the Aussies, Pulisic entered carrying some discomfort; he exited with a true hamstring injury after multiple hard fouls in less than 30 minutes.So, what if head coach Mauricio Pochettino and his staff do decide to rest Pulisic? What if he can’t face the Socceroos?