The Manly Sea Eagles will add another player to a lengthening injury ward to start the season, with young centre and second rower Ben Trbojevic set to be ruled out for a month.

While it's unclear how he picked up an injury, it has been reported the youngster - who was set to gain a spot in the side following other injuries - will miss four weeks.

That puts his return at somewhere between Round 3 and Round 4, with the first game under two weeks away.

The injury to Trbojevic is hardly ideal for Manly, who were already set to be without Josh Schuster for the start of the season following a pre-season training injury. He could miss over a month as he recovers from a syndesmosis injury.

Trbojevic's injury may not have been reported during Friday evening's trial with the Canberra Raiders, but it doesn't mean Manly don't have other problems.

Schuster's starting second row partner Haumole Olokau'atu did an arm injury which is set to rule him out for the season start.

He was joined by Martin Taupau, who jarred his back and will also be racing the clock, while some concern was also handed the way of Dylan Walker who left the field during the game.

It's the second row where Manly could now find themselves disastrously short though.

Should Olokau'atu, Schuster and Trbojevic all be missing, it likely means Sean Keppie and Karl Lawton will start on the edge, with only Andrew Davey or stop-gap solution Ethan Bullemor likely to be able to fill a role on the bench to cover for the edge players.

The Sea Eagles also have the shortest turnaround window to the start of their season, playing the opener against the Penrith Panthers on March 10 in what will be an enormous clash.