The incident, which drew a dangerous contact charge, took place in the 53rd minute of the Sea Eagles' disappointing 20-18 loss at 4 Pines Park on Thursday night.
Feagai was assisted from the field by two trainers and did not return. He will go for scans on a possible serious leg injury that could sideline him long-term.
Sipley was sin-binned and put on report and also left the field for an HIA after the tackle.
Sea Eagles captain Daly Cherry-Evans said post-match Sipley "might have been concussed and then lost his legs" while making the tackle.
"It looks pretty accidental from what I saw," he said.
Manly had their misery compounded during the match with injuries to Nathan Brown (shoulder), Jake Trbojevic (concussion) and Tommy Talau (leg).
The trio are expected to miss the round-eight clash with Penrith.
Coach Anthony Seibold also confirmed the side would be without hooker Lachlan Croker for a month after knee surgery.
The Sea Eagles have now lost their last three matches to slide out of the top eight after being second in round four.
Seibold said star fullback Tom Trbojevic was on track to return against the Panthers at CommBank Stadium next Saturday night.
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