The Republic of Ireland forward, who plays for his nation’s Under-21s, has returned t o the club from Portsmouth.
O’Mahony had been at Pompey since the start of the season, getting regular game time.
Portsmouth boss John Mousinho said: “Mark is going to be out for four to five weeks.
“He has picked up a strain in one of his oblique muscles, which is a pretty rare injury for a footballer.
‘He picked it up against Swansea, obviously, when he came on.
“He has been back at Brighton, he has had a scan and continues to do his rehab back at Brighton.
“We’re in constant communication with them and, as soon as he’s back and he’s available to train, he’ll be back.”
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