St George Illawarra Dragons co-captain Clint Gutherson experienced a hamstring injury during the match against the Cowboys.
The club has also updated the status of several other players.
Buchanan faced a head injury in the Round 4 game on the Gold Coast and missed Round 5. He is currently working on his recovery to return to the field.
Egan is in the final phase of his ACL rehabilitation, sustaining the injury in Round 11 last year. The 21-year-old is back in full training and targeting a return between Rounds 8 and 10.
Tsougranis sustained an ankle syndesmosis injury in the Round 3 Knock-On Effect NSW Cup victory and is expected to return by Round 10.
O'Neill is healing from a pectoral strain and aims to be back on the field around Rounds 7 or 8.