Koula pulled up sore with a knee injury after the Sea Eagles' golden-point win over North Queensland on Saturday night and is now expected to miss close to four weeks.
The excitement machine had only just been moved to the fullback role, in order to allow Trbojevic to return from a hamstring injury at centre.
Koula had been among Manly's best in the win, running for 289 metres and busting seven tackles, setting up a second-half try with a bust down field.
Manly do have options at the back.
Lehi Hopoate had impressed at the back in the month prior to Trbojevic's return, and came off the bench as a utility on Saturday night.
It is possible coach Anthony Seibold will stick him back in the No.1 jersey, given his plan was for Trbojevic to play centre for at least a month.
Manly were also hopeful winger Jason Saab will be fit to face Newcastle on Sunday, after his calf issues were only believed to be cramps against the Cowboys.
The Sea Eagles could require other changes in their backline this week, with halfback Daly Cherry-Evans to miss the Knights clash while in State of Origin camp.
Manly did play winger Tommy Talau in the halves in their last Origin affected match.
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