After surprisingly retiring from NFL, Hayne was used sparingly as defending champions Fiji qualified for the Cup quarter-finals in London on Saturday.
Hayne came off the bench in Fiji's shock 31-10 loss at Twickenham and the two-time NRL Dally M medallist featured in wins over Wales (42-5) and Australia (26-0) later in the day.
The 28-year-old looked off the pace, having only linked up with the squad on Monday, and Fiji coach Ben Ryan admitted Hayne faces an uphill task to secure his spot on the plane to Rio in August.
"His fitness is way down on our boys and that's why we're moderating what he's doing, he was pretty sore come Friday after consecutive training in what's been a light week for us," Ryan said.
"I'd be an idiot of a coach if having played five minutes in the World Series I can start to make assumptions on where he can go.
"He's a good footballer but we've got world class players, we're the reigning world champions.
"When we go into the Olympic camp there's no hiding place, we don't pick by face, we pick by form and he'll have six or seven weeks to prove himself.
"If he gets into our side because of form in seven weeks' time then he's done remarkably well and if he doesn't it just shows the quality we've got."