Seibold may be left wondering where he draws a team from for next week's meeting with South Sydney after a chaotic Sunday afternoon in front of 17,187 spectators at Brookvale Oval.
The Sea Eagles lost Reuben Garrick to concussion along with props Taniela Paseka (thumb) and Toff Sipley (knee) in the first half.
Utility forward Corey Waddell played through considerable pain with a lower leg injury as the Sea Eagles hung on to claim just their second win since May and climb up to eighth spot on the NRL ladder.
For the Dragons, it was a case of one step forward, another step back after failing to make their numerical advantage count.
The Red V's afternoon got off to a less-than-ideal start when Moses Suli pulled out of the warm-up with a hamstring issue, forcing Mat Feagai into the centres.
Manly immediately identified the weakness, with fullback Lehi Hopoate slipping past Feagai to put the home side up after just four minutes.
The Sea Eagles lost Paseka to start their afternoon of misfortune, with Manly staff unable to put the big prop's thumb back in its place.
With Paseka off the field, the Dragons took the lead as Jacob Liddle and Kyle Flanagan crossed for tries in quick succession.
Flanagan's four-pointer came while both Sipley and Garrick were down for treatment, but the depleted Sea Eagles were able to respond when Haumole Olakau'atu trampled his way to the tryline in the 25th minute.
Garrick, who had only just returned from his first head injury assessment, was sent for a second test before halftime and was automatically ruled out of a return after stumbling out of a collision.
It meant the Sea Eagles began the second half with just 14 fit players, with Waddell limping through the discomfort to punch out an 18-minute second-half stint.
Luke Brooks put Manly ahead, with Zac Lomax responding for the Dragons, levelling the scores at 14-14.
But rather than fold, Manly came alive, with Ben Trbojevic gliding in on the right edge to push the home side ahead.
That confidence only continued to grow when his big brother - and newly minted NSW State of Origin captain - Jake collected a Daly Cherry-Evans grubber kick to give Manly a 10-point buffer.
Cherry-Evans showed his class once more, chipping behind the Dragons defence for Jason Saab to touch down and seal the win.
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