The Newcastle United striker is in O'Neill's Republic of Ireland squad for Friday night's World Cup qualifier against Wales in Dublin.
Murphy, however, sat out this morning's training session.
The 34-year-old – who played in Saturday's goalless draw against Birmingham City – has a calf problem.
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O'Neill said: "Daryl Murphy is giving himself a chance. He's not ruled out yet.
"We'll see what happens with the scan."
Meanwhile, defender Grant Hanley has been forced out of the Scotland squad with an ankle injury suffered at St Andrew's.