The 30th-minute incident saw former Tigers, now Sea Eagles prop Josh Aloiai placed on report for a cannonball tackle.
The incident left Seyfarth unable to continue in the game, carried from the field and eventually sighted in a knee brace.
The 22-year-old Tigers' bench option, only managed five minutes in last night's contest, which was his 11th appearance of the season. A consistent option in recent weeks for Tigers coach Michael Maguire, the youngster could well have played his last game for the season.
Tigers staff confirmed he had sustained an MCL injury from the incident, and with only six weeks to go in the season, it's likely he will need to recover ahead of schedule to play again.
Aloiai was placed on report for the hit and will face a nervous wait for the NRL's charge sheet to come out, despite avoiding a trip to the sin bin.
Maguire hinted that the 18th man rule could be tweaked to allow it to be activated in circumstances such as last night, despite Aloiai not going to the sin bin. The Tigers' 18th man against Manly was Michael Chee-Kam.
"I want to review the tackle before I make a judgement on (whether there should be a case for the 18th man to be activated) that but we lose a player in that circumstance and you've got the 18th man there," Maguire said.
"We can use the 18th man around the concussion rule but in this case we'll have to have a look at it, because he's got to get 10 minutes in the bin to ignite the 18th man."
The Tigers also lost second rower Luke Garner to an ankle injury during the contest, leaving them with just 15 for the entire second half.
The severity of Garner's injury is unclear at this stage.
Michael Maguire's side lost the contest 44-24, making it the fourth time in five weeks they have let in 40 or more points.