Scans revealed that Stewart sustained a chipped bone after taking a blow during the first half of the match at Kogarah on Sunday.
“I believe late scans yesterday, he had first scans, which showed nothing, but we had to send him back to the doctor,” head coach Geoff Toovey told Manly’s official website.
“The latest scans showed that he’s got a small chip in his back, very similar to the problem that Tony Williams had last year.”
“He’ll be out for probably about a month or so.”
The injury rules the 28-year-old out of contention for a NSW Origin spot in the opening match of the series on June 5.
“There’s no dramas,” added Toovey.
“It’s not physically threatening or anything like that, it’s just one of those things that irritates the system and needs time to settle down.”
“He’s pretty positive about it. I suppose he’s walking around and he seems quite well.”