The former Cowboys hooker joined the club for 2017 but injured his throat in an accident during training on Saturday.
"Kostjasyn was hospitalised however he has been discharged today (Monday)," the Knights wrote on their website.
"The 29-year-old will be reassessed in four to six weeks to determine a return date to training. At this stage, Kostjasyn is expected to be out for a minimum of six to eight weeks.
"The Club will continue to provide any further updates when appropriate."
The 2015 grand final winner is a major recruit for the wooden spooners who also signed Jamie Buhrer from Manly.